diff --git a/src/main/java/io/weaviate/client6/v1/api/collections/data/InsertManyRequest.java b/src/main/java/io/weaviate/client6/v1/api/collections/data/InsertManyRequest.java
index c8a12fa0..b4d86cfb 100644
--- a/src/main/java/io/weaviate/client6/v1/api/collections/data/InsertManyRequest.java
+++ b/src/main/java/io/weaviate/client6/v1/api/collections/data/InsertManyRequest.java
@@ -5,11 +5,11 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.UUID;
-import java.util.stream.Collectors;
 
 import io.weaviate.client6.v1.api.collections.CollectionHandleDefaults;
 import io.weaviate.client6.v1.api.collections.ObjectMetadata;
 import io.weaviate.client6.v1.api.collections.WeaviateObject;
+import io.weaviate.client6.v1.internal.MapUtil;
 import io.weaviate.client6.v1.internal.grpc.ByteStringUtil;
 import io.weaviate.client6.v1.internal.grpc.Rpc;
 import io.weaviate.client6.v1.internal.grpc.protocol.WeaviateGrpc.WeaviateBlockingStub;
@@ -65,8 +65,10 @@ public static <T> Rpc<InsertManyRequest<T>, WeaviateProtoBatch.BatchObjectsReque
           var errors = new ArrayList<String>(insertErrors.size());
           var uuids = new ArrayList<String>();
 
-          var failed = insertErrors.stream()
-              .collect(Collectors.toMap(err -> err.getIndex(), err -> err.getError()));
+          var failed = MapUtil.collect(
+              insertErrors.stream(),
+              err -> err.getIndex(),
+              err -> err.getError());
 
           var iter = insertObjects.listIterator();
           while (iter.hasNext()) {
@@ -137,6 +139,10 @@ public static <T> void buildObject(WeaviateProtoBatch.BatchObject.Builder object
           var value = entry.getValue();
           var protoValue = com.google.protobuf.Value.newBuilder();
 
+          if (value == null) {
+            return;
+          }
+
           if (value instanceof String v) {
             protoValue.setStringValue(v);
           } else if (value instanceof UUID v) {
@@ -147,7 +153,7 @@ public static <T> void buildObject(WeaviateProtoBatch.BatchObject.Builder object
             protoValue.setBoolValue(v.booleanValue());
           } else if (value instanceof Number v) {
             protoValue.setNumberValue(v.doubleValue());
-          } else if (value instanceof List v) {
+          } else if (value instanceof List<?> v) {
             protoValue.setListValue(
                 com.google.protobuf.ListValue.newBuilder()
                     .addAllValues(v.stream()
diff --git a/src/main/java/io/weaviate/client6/v1/api/collections/query/QueryRequest.java b/src/main/java/io/weaviate/client6/v1/api/collections/query/QueryRequest.java
index 9f42be01..cb9daa54 100644
--- a/src/main/java/io/weaviate/client6/v1/api/collections/query/QueryRequest.java
+++ b/src/main/java/io/weaviate/client6/v1/api/collections/query/QueryRequest.java
@@ -88,7 +88,13 @@ static <T> Rpc<QueryRequest, WeaviateProtoSearchGet.SearchRequest, QueryResponse
                     group.getMaxDistance(),
                     group.getNumberOfObjects(),
                     objects);
-              }).collect(Collectors.toMap(QueryResponseGroup::name, Function.identity()));
+              })
+              // Collectors.toMap() throws an NPE if either key or value in the map are null.
+              // In this specific case it is safe to use it, as the function in the map above
+              // always returns a QueryResponseGroup.
+              // The name of the group should not be null either, that's something we assume
+              // about the server's response.
+              .collect(Collectors.toMap(QueryResponseGroup::name, Function.identity()));
 
           return new QueryResponseGrouped<T>(allObjects, groups);
         }, () -> rpc.method(), () -> rpc.methodAsync());
diff --git a/src/main/java/io/weaviate/client6/v1/internal/MapUtil.java b/src/main/java/io/weaviate/client6/v1/internal/MapUtil.java
new file mode 100644
index 00000000..4186572b
--- /dev/null
+++ b/src/main/java/io/weaviate/client6/v1/internal/MapUtil.java
@@ -0,0 +1,42 @@
+package io.weaviate.client6.v1.internal;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public final class MapUtil {
+  /** Prevent public initialization. */
+  private MapUtil() {
+  }
+
+  /**
+   * Collect stream entries into a map. Use this method whenever
+   * potential null keys or null values prohibit {@link Collectors#toMap}.
+   *
+   * <p>
+   * Example:
+   *
+   * <pre>{@code
+   * Map<Integer, Integer> = MapUtil.collect(
+   *  Stream.of(1, 2, 3),
+   *  Function.identity(), // use value as key
+   *  el -> el.equals(3) ? null : el;
+   * );
+   *
+   * // Result: {1: 1, 2: 2, 3: null};
+   * }</pre>
+   *
+   * @param stream Stream of elements {@link T}.
+   * @param keyFn  Transforms element {@link T} to key {@link K}.
+   * @param keyFn  Transforms element {@link T} to value {@link V}.
+   * @return Map
+   */
+  public static <K, V, T> Map<K, V> collect(Stream<T> stream, Function<T, K> keyFn, Function<T, V> valueFn) {
+    return stream.collect(
+        HashMap::new,
+        (m, el) -> m.put(keyFn.apply(el), valueFn.apply(el)),
+        HashMap::putAll);
+  }
+}
diff --git a/src/main/java/io/weaviate/client6/v1/internal/orm/MapReader.java b/src/main/java/io/weaviate/client6/v1/internal/orm/MapReader.java
index dcf0af65..004eb0a4 100644
--- a/src/main/java/io/weaviate/client6/v1/internal/orm/MapReader.java
+++ b/src/main/java/io/weaviate/client6/v1/internal/orm/MapReader.java
@@ -1,16 +1,19 @@
 package io.weaviate.client6.v1.internal.orm;
 
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 
 public class MapReader implements PropertiesReader<Map<String, Object>> {
   private final Map<String, Object> properties;
 
   public MapReader(Map<String, Object> properties) {
-    this.properties = properties;
+    // Defensive copy to ensure original properties are not modified
+    this.properties = Collections.unmodifiableMap(new HashMap<>(properties));
   }
 
   @Override
   public Map<String, Object> readProperties() {
-    return Map.copyOf(properties); // ensure original properties immutable
+    return properties;
   }
 }