diff --git a/packages/core/src/collection/collection.persistent.ts b/packages/core/src/collection/collection.persistent.ts
index f7425ff1..86b049a3 100644
--- a/packages/core/src/collection/collection.persistent.ts
+++ b/packages/core/src/collection/collection.persistent.ts
@@ -156,7 +156,7 @@ export class CollectionPersistent<
 
             // If successfully loaded Item value, assign Item to Collection
             if (loadedPersistedValueIntoItem)
-              this.collection().assignItem(dummyItem); // TODO SECOND GROUP REBUILD (by calling rebuildGroupThatInclude() in the assignItem() method)
+              this.collection().assignItem(dummyItem, { overwrite: false }); // TODO SECOND GROUP REBUILD (by calling rebuildGroupThatInclude() in the assignItem() method)
           }
         }
       }
diff --git a/packages/core/src/collection/index.ts b/packages/core/src/collection/index.ts
index edf12d81..e4aec4b3 100644
--- a/packages/core/src/collection/index.ts
+++ b/packages/core/src/collection/index.ts
@@ -193,7 +193,7 @@ export class Collection<DataType extends Object = DefaultItem> {
    * @param config - Configuration object
    */
   public Selector(
-    initialKey: ItemKey,
+    initialKey: ItemKey | null,
     config: SelectorConfigInterface = {}
   ): Selector<DataType> {
     if (this.isInstantiated) {
@@ -489,7 +489,7 @@ export class Collection<DataType extends Object = DefaultItem> {
    * @param config - Configuration object
    */
   public getGroup(
-    groupKey: GroupKey | undefined,
+    groupKey: GroupKey | undefined | null,
     config: HasConfigInterface = {}
   ): Group<DataType> | undefined {
     config = defineConfig(config, {
@@ -589,7 +589,7 @@ export class Collection<DataType extends Object = DefaultItem> {
    */
   public createSelector(
     selectorKey: SelectorKey,
-    itemKey: ItemKey
+    itemKey: ItemKey | null
   ): Selector<DataType> {
     let selector = this.getSelector(selectorKey, { notExisting: true });
     if (!this.isInstantiated) LogCodeManager.log('1B:02:04');
@@ -662,7 +662,7 @@ export class Collection<DataType extends Object = DefaultItem> {
    * @param config - Configuration object
    */
   public getSelector(
-    selectorKey: SelectorKey | undefined,
+    selectorKey: SelectorKey | undefined | null,
     config: HasConfigInterface = {}
   ): Selector<DataType> | undefined {
     config = defineConfig(config, {
@@ -699,14 +699,10 @@ export class Collection<DataType extends Object = DefaultItem> {
 
     // Create dummy Selector to hold reference
     if (selector == null) {
-      selector = new Selector<DataType>(
-        this,
-        Selector.unknownItemPlaceholderKey,
-        {
-          key: selectorKey,
-          isPlaceholder: true,
-        }
-      );
+      selector = new Selector<DataType>(this, null, {
+        key: selectorKey,
+        isPlaceholder: true,
+      });
       this.selectors[selectorKey] = selector;
     }
 
@@ -773,7 +769,7 @@ export class Collection<DataType extends Object = DefaultItem> {
    * @param config - Configuration object
    */
   public getItem(
-    itemKey: ItemKey | undefined,
+    itemKey: ItemKey | undefined | null,
     config: HasConfigInterface = {}
   ): Item<DataType> | undefined {
     config = defineConfig(config, {
@@ -1425,9 +1421,11 @@ export class Collection<DataType extends Object = DefaultItem> {
     }
 
     // Check if Item already exists
-    if (this.getItem(itemKey) != null) {
-      if (!config.overwrite) return true;
-      else increaseCollectionSize = false;
+    if (this.getItem(itemKey, { notExisting: true }) != null) {
+      if (!config.overwrite) {
+        this.assignData(item._value);
+        return true;
+      } else increaseCollectionSize = false;
     }
 
     // Assign/add Item to Collection
diff --git a/packages/core/src/collection/selector.ts b/packages/core/src/collection/selector.ts
index 1366f24d..5e484e9d 100644
--- a/packages/core/src/collection/selector.ts
+++ b/packages/core/src/collection/selector.ts
@@ -8,20 +8,19 @@ import {
   StateRuntimeJobConfigInterface,
 } from '../internal';
 
-export class Selector<DataType extends Object = DefaultItem> extends State<
-  DataType | undefined
-> {
+export class Selector<
+  DataType extends Object = DefaultItem
+> extends State<DataType | null> {
   // Collection the Selector belongs to
   public collection: () => Collection<DataType>;
 
-  static unknownItemPlaceholderKey = '__UNKNOWN__ITEM__KEY__';
   static rebuildSelectorSideEffectKey = 'rebuildSelector';
   static rebuildItemSideEffectKey = 'rebuildItem';
 
   // Item the Selector represents
-  public _item: Item<DataType> | undefined;
+  public _item: Item<DataType> | null;
   // Key/Name identifier of the Item the Selector represents
-  public _itemKey: ItemKey;
+  public _itemKey: ItemKey | null;
 
   /**
    * A Selector represents an Item from a Collection in the long term.
@@ -39,23 +38,21 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    */
   constructor(
     collection: Collection<DataType>,
-    itemKey: ItemKey,
+    itemKey: ItemKey | null,
     config: SelectorConfigInterface = {}
   ) {
     config = defineConfig(config, {
       isPlaceholder: false,
     });
-    super(collection.agileInstance(), undefined, config);
+    super(collection.agileInstance(), null, config);
     this.collection = () => collection;
-    this._item = undefined;
-    this._itemKey = !config.isPlaceholder
-      ? itemKey
-      : Selector.unknownItemPlaceholderKey;
+    this._item = null;
+    this._itemKey = !config.isPlaceholder && itemKey != null ? itemKey : null;
     this._key = config?.key;
     this.isPlaceholder = true; // Because hasn't selected any Item yet
 
     // Initial select of the Item
-    if (!config.isPlaceholder) this.select(itemKey, { overwrite: true });
+    if (this._itemKey != null) this.select(itemKey, { overwrite: true });
   }
 
   /**
@@ -65,7 +62,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    *
    * @public
    */
-  public get itemKey(): ItemKey {
+  public get itemKey(): ItemKey | null {
     return this._itemKey;
   }
 
@@ -78,7 +75,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    * @public
    * @param value - New key/name identifier of the Item to be represented by the Selector.
    */
-  public set itemKey(value: ItemKey) {
+  public set itemKey(value: ItemKey | null) {
     this.select(value);
   }
 
@@ -89,7 +86,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    *
    * @public
    */
-  public get item(): Item<DataType> | undefined {
+  public get item(): Item<DataType> | null {
     return this._item;
   }
 
@@ -102,7 +99,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    * @public
    * @param value - New Item to be represented by the Selector.
    */
-  public set item(value: Item<DataType> | undefined) {
+  public set item(value: Item<DataType> | null) {
     if (value?._key) this.select(value._key);
   }
 
@@ -116,7 +113,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    * @param config - Configuration object
    */
   public select(
-    itemKey: ItemKey,
+    itemKey: ItemKey | null,
     config: StateRuntimeJobConfigInterface = {}
   ): this {
     config = defineConfig(config, {
@@ -140,7 +137,9 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
       return this;
 
     // Unselect old Item
-    this.unselect({ background: true });
+    this.unselect({ background: itemKey != null });
+
+    if (itemKey == null) return this;
 
     // Retrieve new Item from Collection
     const newItem = this.collection().getItemWithReference(itemKey);
@@ -224,16 +223,20 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
     });
 
     // Unselect Item
-    if (item) {
+    if (item != null) {
       item.selectedBy.delete(this._key as any);
       item.removeSideEffect(Selector.rebuildSelectorSideEffectKey);
       item.removeSideEffect(Selector.rebuildItemSideEffectKey);
-      if (item.isPlaceholder) delete this.collection().data[this._itemKey];
+      if (
+        item.isPlaceholder &&
+        this._itemKey != null
+      )
+        delete this.collection().data[this._itemKey];
     }
 
     // Reset Selector
-    this._item = undefined;
-    this._itemKey = Selector.unknownItemPlaceholderKey;
+    this._item = null;
+    this._itemKey = null;
     this.rebuildSelector(config);
     this.isPlaceholder = true;
 
@@ -250,7 +253,10 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
    * @param itemKey - Key/Name identifier of the Item.
    * @param correctlySelected - Whether the Item has to be selected correctly.
    */
-  public hasSelected(itemKey: ItemKey, correctlySelected = true): boolean {
+  public hasSelected(
+    itemKey: ItemKey | null,
+    correctlySelected = true
+  ): boolean {
     if (correctlySelected) {
       return (
         this._itemKey === itemKey &&
@@ -273,7 +279,7 @@ export class Selector<DataType extends Object = DefaultItem> extends State<
   public rebuildSelector(config: StateRuntimeJobConfigInterface = {}): this {
     // Assign 'undefined' to the Selector value if no Item is set
     if (this._item == null || this._item.isPlaceholder) {
-      this.set(undefined, config);
+      this.set(null, config);
       return this;
     }
 
diff --git a/packages/core/tests/unit/collection/collection.persistent.test.ts b/packages/core/tests/unit/collection/collection.persistent.test.ts
index 764b482c..3ebbc97c 100644
--- a/packages/core/tests/unit/collection/collection.persistent.test.ts
+++ b/packages/core/tests/unit/collection/collection.persistent.test.ts
@@ -414,9 +414,9 @@ describe('CollectionPersistent Tests', () => {
               followCollectionPersistKeyPattern: false,
             }
           );
-          expect(dummyCollection.assignItem).toHaveBeenCalledWith(
-            placeholderItem1
-          );
+          expect(
+            dummyCollection.assignItem
+          ).toHaveBeenCalledWith(placeholderItem1, { overwrite: false });
           expect(dummyCollection.assignItem).not.toHaveBeenCalledWith(
             placeholderItem2
           ); // Because Item persistent isn't ready
@@ -507,9 +507,9 @@ describe('CollectionPersistent Tests', () => {
               followCollectionPersistKeyPattern: false,
             }
           );
-          expect(dummyCollection.assignItem).toHaveBeenCalledWith(
-            placeholderItem1
-          );
+          expect(
+            dummyCollection.assignItem
+          ).toHaveBeenCalledWith(placeholderItem1, { overwrite: false });
           expect(dummyCollection.assignItem).not.toHaveBeenCalledWith(
             placeholderItem3
           ); // Because Item 3 is already present in the Collection
diff --git a/packages/core/tests/unit/collection/collection.test.ts b/packages/core/tests/unit/collection/collection.test.ts
index 3ad578f8..8d709e8d 100644
--- a/packages/core/tests/unit/collection/collection.test.ts
+++ b/packages/core/tests/unit/collection/collection.test.ts
@@ -1414,7 +1414,10 @@ describe('Collection Tests', () => {
 
         expect(response).toBeInstanceOf(Selector);
         expect(response.isPlaceholder).toBeTruthy();
+        expect(response._item).toBeNull();
+        expect(response._itemKey).toBeNull();
         expect(response._key).toBe('notExistingSelector');
+
         expect(collection.selectors['notExistingSelector']).toBe(response);
         expect(ComputedTracker.tracked).toHaveBeenCalledWith(response.observer);
       });
@@ -2832,6 +2835,7 @@ describe('Collection Tests', () => {
         dummyItem1.patch = jest.fn();
         toAddDummyItem2.patch = jest.fn();
         collection.rebuildGroupsThatIncludeItemKey = jest.fn();
+        collection.assignData = jest.fn();
       });
 
       it('should assign valid Item to Collection (default config)', () => {
@@ -2847,6 +2851,7 @@ describe('Collection Tests', () => {
             background: false,
           }
         );
+        expect(collection.assignData).not.toHaveBeenCalled();
 
         expect(toAddDummyItem2.patch).not.toHaveBeenCalled();
         expect(toAddDummyItem2._key).toBe('dummyItem2');
@@ -2870,6 +2875,7 @@ describe('Collection Tests', () => {
             background: true,
           }
         );
+        expect(collection.assignData).not.toHaveBeenCalled();
 
         expect(toAddDummyItem2.patch).not.toHaveBeenCalled();
         expect(toAddDummyItem2._key).toBe('dummyItem2');
@@ -2895,6 +2901,7 @@ describe('Collection Tests', () => {
             background: false,
           }
         );
+        expect(collection.assignData).not.toHaveBeenCalled();
 
         expect(toAddDummyItem2.patch).toHaveBeenCalledWith(
           { id: 'randomDummyId' },
@@ -2923,6 +2930,7 @@ describe('Collection Tests', () => {
         expect(
           collection.rebuildGroupsThatIncludeItemKey
         ).not.toHaveBeenCalled();
+        expect(collection.assignData).not.toHaveBeenCalled();
 
         expect(toAddDummyItem2.patch).not.toHaveBeenCalled();
         expect(toAddDummyItem2._key).toBe('dummyItem2');
@@ -2944,6 +2952,7 @@ describe('Collection Tests', () => {
         expect(
           collection.rebuildGroupsThatIncludeItemKey
         ).not.toHaveBeenCalled();
+        expect(collection.assignData).toHaveBeenCalledWith(dummyItem1._value);
 
         expect(dummyItem1.patch).not.toHaveBeenCalled();
         expect(dummyItem1._key).toBe('dummyItem1');
@@ -2965,6 +2974,7 @@ describe('Collection Tests', () => {
             background: false,
           }
         );
+        expect(collection.assignData).not.toHaveBeenCalled();
 
         expect(dummyItem1.patch).not.toHaveBeenCalled();
         expect(dummyItem1._key).toBe('dummyItem1');
diff --git a/packages/core/tests/unit/collection/selector.test.ts b/packages/core/tests/unit/collection/selector.test.ts
index cb4b2f87..96e6170b 100644
--- a/packages/core/tests/unit/collection/selector.test.ts
+++ b/packages/core/tests/unit/collection/selector.test.ts
@@ -29,20 +29,21 @@ describe('Selector Tests', () => {
     const selector = new Selector(dummyCollection, 'dummyItemKey');
 
     expect(selector.collection()).toBe(dummyCollection);
-    expect(selector._item).toBeUndefined();
+    expect(selector._item).toBeNull(); // Because 'select()' is mocked
     expect(selector._itemKey).toBe('dummyItemKey');
     expect(selector.select).toHaveBeenCalledWith('dummyItemKey', {
       overwrite: true,
     });
 
+    // Check if State was called with correct parameters
     expect(selector._key).toBeUndefined();
     expect(selector.valueType).toBeUndefined();
     expect(selector.isSet).toBeFalsy();
     expect(selector.isPlaceholder).toBeTruthy();
-    expect(selector.initialStateValue).toBeUndefined();
-    expect(selector._value).toBeUndefined();
-    expect(selector.previousStateValue).toBeUndefined();
-    expect(selector.nextStateValue).toBeUndefined();
+    expect(selector.initialStateValue).toBeNull();
+    expect(selector._value).toBeNull();
+    expect(selector.previousStateValue).toBeNull();
+    expect(selector.nextStateValue).toBeNull();
     expect(selector.observer).toBeInstanceOf(StateObserver);
     expect(selector.observer.dependents.size).toBe(0);
     expect(selector.observer._key).toBeUndefined();
@@ -65,20 +66,21 @@ describe('Selector Tests', () => {
     });
 
     expect(selector.collection()).toBe(dummyCollection);
-    expect(selector._item).toBeUndefined();
+    expect(selector._item).toBeNull(); // Because 'select()' is mocked
     expect(selector._itemKey).toBe('dummyItemKey');
     expect(selector.select).toHaveBeenCalledWith('dummyItemKey', {
       overwrite: true,
     });
 
+    // Check if State was called with correct parameters
     expect(selector._key).toBe('dummyKey');
     expect(selector.valueType).toBeUndefined();
     expect(selector.isSet).toBeFalsy();
     expect(selector.isPlaceholder).toBeTruthy();
-    expect(selector.initialStateValue).toBeUndefined();
-    expect(selector._value).toBeUndefined();
-    expect(selector.previousStateValue).toBeUndefined();
-    expect(selector.nextStateValue).toBeUndefined();
+    expect(selector.initialStateValue).toBeNull();
+    expect(selector._value).toBeNull();
+    expect(selector.previousStateValue).toBeNull();
+    expect(selector.nextStateValue).toBeNull();
     expect(selector.observer).toBeInstanceOf(StateObserver);
     expect(selector.observer.dependents.size).toBe(0);
     expect(selector.observer._key).toBe('dummyKey');
@@ -101,18 +103,52 @@ describe('Selector Tests', () => {
     });
 
     expect(selector.collection()).toBe(dummyCollection);
-    expect(selector._item).toBeUndefined();
-    expect(selector._itemKey).toBe(Selector.unknownItemPlaceholderKey);
+    expect(selector._item).toBeNull();
+    expect(selector._itemKey).toBeNull();
     expect(selector.select).not.toHaveBeenCalled();
 
+    // Check if State was called with correct parameters
     expect(selector._key).toBeUndefined();
     expect(selector.valueType).toBeUndefined();
     expect(selector.isSet).toBeFalsy();
     expect(selector.isPlaceholder).toBeTruthy();
-    expect(selector.initialStateValue).toBeUndefined();
-    expect(selector._value).toBeUndefined();
-    expect(selector.previousStateValue).toBeUndefined();
-    expect(selector.nextStateValue).toBeUndefined();
+    expect(selector.initialStateValue).toBeNull();
+    expect(selector._value).toBeNull();
+    expect(selector.previousStateValue).toBeNull();
+    expect(selector.nextStateValue).toBeNull();
+    expect(selector.observer).toBeInstanceOf(StateObserver);
+    expect(selector.observer.dependents.size).toBe(0);
+    expect(selector.observer._key).toBeUndefined();
+    expect(selector.sideEffects).toStrictEqual({});
+    expect(selector.computeValueMethod).toBeUndefined();
+    expect(selector.computeExistsMethod).toBeInstanceOf(Function);
+    expect(selector.isPersisted).toBeFalsy();
+    expect(selector.persistent).toBeUndefined();
+    expect(selector.watchers).toStrictEqual({});
+  });
+
+  it("should create Selector and shouldn't call initial select if specified selector key is null (default config)", () => {
+    // Overwrite select once to not call it
+    jest
+      .spyOn(Selector.prototype, 'select')
+      .mockReturnValueOnce(undefined as any);
+
+    const selector = new Selector(dummyCollection, null);
+
+    expect(selector.collection()).toBe(dummyCollection);
+    expect(selector._item).toBeNull();
+    expect(selector._itemKey).toBeNull();
+    expect(selector.select).not.toHaveBeenCalled();
+
+    // Check if State was called with correct parameters
+    expect(selector._key).toBeUndefined();
+    expect(selector.valueType).toBeUndefined();
+    expect(selector.isSet).toBeFalsy();
+    expect(selector.isPlaceholder).toBeTruthy();
+    expect(selector.initialStateValue).toBeNull();
+    expect(selector._value).toBeNull();
+    expect(selector.previousStateValue).toBeNull();
+    expect(selector.nextStateValue).toBeNull();
     expect(selector.observer).toBeInstanceOf(StateObserver);
     expect(selector.observer.dependents.size).toBe(0);
     expect(selector.observer._key).toBeUndefined();
@@ -236,8 +272,9 @@ describe('Selector Tests', () => {
           expect.any(Function),
           { weight: 100 }
         );
-        expect(dummyItem2.selectedBy.size).toBe(1);
-        expect(dummyItem2.selectedBy.has(selector._key as any));
+        expect(Array.from(dummyItem2.selectedBy)).toStrictEqual([
+          selector._key,
+        ]);
       });
 
       it('should unselect old selected Item and select new Item (specific config)', () => {
@@ -300,8 +337,9 @@ describe('Selector Tests', () => {
         expect(selector.addSideEffect).not.toHaveBeenCalled();
 
         expect(dummyItem1.addSideEffect).not.toHaveBeenCalled();
-        expect(dummyItem1.selectedBy.size).toBe(1);
-        expect(dummyItem1.selectedBy.has(selector._key as any));
+        expect(Array.from(dummyItem1.selectedBy)).toStrictEqual([
+          selector._key,
+        ]);
       });
 
       it('should select selected Item again (config.force = true)', () => {
@@ -361,7 +399,7 @@ describe('Selector Tests', () => {
         expect(selector.addSideEffect).not.toHaveBeenCalled();
 
         expect(dummyItem2.addSideEffect).not.toHaveBeenCalled();
-        expect(dummyItem2.selectedBy.size).toBe(0);
+        expect(Array.from(dummyItem2.selectedBy)).toStrictEqual([]);
       });
 
       it("should unselect old selected Item and select new Item although Collection isn't instantiated (config.force = true)", () => {
@@ -404,8 +442,9 @@ describe('Selector Tests', () => {
           expect.any(Function),
           { weight: 100 }
         );
-        expect(dummyItem2.selectedBy.size).toBe(1);
-        expect(dummyItem2.selectedBy.has(selector._key as any));
+        expect(Array.from(dummyItem2.selectedBy)).toStrictEqual([
+          selector._key,
+        ]);
       });
 
       it('should remove old selected Item, select new Item and overwrite Selector if old Item is placeholder (default config)', async () => {
@@ -445,8 +484,9 @@ describe('Selector Tests', () => {
           expect.any(Function),
           { weight: 100 }
         );
-        expect(dummyItem2.selectedBy.size).toBe(1);
-        expect(dummyItem2.selectedBy.has(selector._key as any));
+        expect(Array.from(dummyItem2.selectedBy)).toStrictEqual([
+          selector._key,
+        ]);
       });
 
       it("should remove old selected Item, select new Item and shouldn't overwrite Selector if old Item is placeholder (config.overwrite = false)", async () => {
@@ -486,8 +526,22 @@ describe('Selector Tests', () => {
           expect.any(Function),
           { weight: 100 }
         );
-        expect(dummyItem2.selectedBy.size).toBe(1);
-        expect(dummyItem2.selectedBy.has(selector._key as any));
+        expect(Array.from(dummyItem2.selectedBy)).toStrictEqual([
+          selector._key,
+        ]);
+      });
+
+      it("shouldn't select null and unselect old Item (default config)", () => {
+        dummyCollection.getItemWithReference = jest.fn(() => dummyItem2);
+
+        selector.select(null);
+
+        expect(dummyCollection.getItemWithReference).not.toHaveBeenCalled();
+        // expect(selector._itemKey).toBe(Selector.unknownItemPlaceholderKey); // Because 'unselect()' is mocked
+        // expect(selector._item).toBeNull(); // Because 'unselect()' is mocked
+        expect(selector.unselect).toHaveBeenCalledWith({ background: false });
+        expect(selector.rebuildSelector).not.toHaveBeenCalled();
+        expect(selector.addSideEffect).not.toHaveBeenCalled();
       });
 
       describe('test added sideEffect called Selector.rebuildSelectorSideEffectKey', () => {
@@ -613,8 +667,8 @@ describe('Selector Tests', () => {
           Selector.rebuildSelectorSideEffectKey
         );
 
-        expect(selector._item).toBeUndefined();
-        expect(selector._itemKey).toBe(Selector.unknownItemPlaceholderKey);
+        expect(selector._item).toBeNull();
+        expect(selector._itemKey).toBeNull();
         expect(selector.isPlaceholder).toBeTruthy();
         expect(selector.rebuildSelector).toHaveBeenCalledWith({});
 
@@ -633,8 +687,8 @@ describe('Selector Tests', () => {
           Selector.rebuildSelectorSideEffectKey
         );
 
-        expect(selector._item).toBeUndefined();
-        expect(selector._itemKey).toBe(Selector.unknownItemPlaceholderKey);
+        expect(selector._item).toBeNull();
+        expect(selector._itemKey).toBeNull();
         expect(selector.isPlaceholder).toBeTruthy();
         expect(selector.rebuildSelector).toHaveBeenCalledWith({
           background: true,
@@ -655,8 +709,8 @@ describe('Selector Tests', () => {
           Selector.rebuildSelectorSideEffectKey
         );
 
-        expect(selector._item).toBeUndefined();
-        expect(selector._itemKey).toBe(Selector.unknownItemPlaceholderKey);
+        expect(selector._item).toBeNull();
+        expect(selector._itemKey).toBeNull();
         expect(selector.isPlaceholder).toBeTruthy();
         expect(selector.rebuildSelector).toHaveBeenCalledWith({});
 
@@ -684,13 +738,13 @@ describe('Selector Tests', () => {
       });
 
       it("should return false if Selector hasn't selected itemKey correctly (item = undefined)", () => {
-        selector._item = undefined;
+        selector._item = null;
 
         expect(selector.hasSelected('dummyItemKey')).toBeFalsy();
       });
 
       it("should return true if Selector hasn't selected itemKey correctly (item = undefined, correctlySelected = false)", () => {
-        selector._item = undefined;
+        selector._item = null;
 
         expect(selector.hasSelected('dummyItemKey', false)).toBeTruthy();
       });
@@ -742,11 +796,11 @@ describe('Selector Tests', () => {
       });
 
       it('should set selector value to undefined if Item is undefined (default config)', () => {
-        selector._item = undefined;
+        selector._item = null;
 
         selector.rebuildSelector();
 
-        expect(selector.set).toHaveBeenCalledWith(undefined, {});
+        expect(selector.set).toHaveBeenCalledWith(null, {});
       });
     });
   });
diff --git a/packages/core/tests/unit/state/state.observer.test.ts b/packages/core/tests/unit/state/state.observer.test.ts
index 7a248cb8..ac95ca58 100644
--- a/packages/core/tests/unit/state/state.observer.test.ts
+++ b/packages/core/tests/unit/state/state.observer.test.ts
@@ -133,7 +133,7 @@ describe('StateObserver Tests', () => {
       it(
         "should call 'ingestValue' with computed value " +
           'if Observer belongs to a Computed State (default config)',
-        () => {
+        async () => {
           dummyComputed.compute = jest.fn(() =>
             Promise.resolve('computedValue')
           );
@@ -141,7 +141,7 @@ describe('StateObserver Tests', () => {
           computedObserver.ingest();
 
           expect(dummyComputed.compute).toHaveBeenCalled();
-          waitForExpect(() => {
+          await waitForExpect(() => {
             expect(computedObserver.ingestValue).toHaveBeenCalledWith(
               'computedValue',
               {}