diff --git a/test/types/models.test.ts b/test/types/models.test.ts
index 22b0916941..0cd4b08c1e 100644
--- a/test/types/models.test.ts
+++ b/test/types/models.test.ts
@@ -1056,3 +1056,31 @@ async function gh16526() {
   const insertManyResult = await Tank.insertMany([{ name: 'test' }], { lean: true, rawResult: true });
   expectType<number>(insertManyResult.insertedCount);
 }
+
+async function gh15693() {
+  interface IUser {
+    name: string;
+  }
+
+  interface UserMethods {
+    printName(this: IUser): void;
+    getName(): string;
+  }
+
+  const schema = new Schema<IUser, Model<IUser>, UserMethods>({ name: { type: String, required: true } });
+  schema.method('printName', function printName(this: IUser) {
+    expectError(this.isModified('name'));
+    expectError(this.doesNotExist());
+    expectType<string>(this.name);
+    console.log(this.name);
+  });
+  schema.method('getName', function getName() {
+    expectType<boolean>(this.isModified('name'));
+    return this.name;
+  });
+  const User = model('user', schema);
+
+  const leanInst = await User.findOne({}).lean().orFail();
+  User.schema.methods.printName.apply(leanInst);
+
+}
diff --git a/types/models.d.ts b/types/models.d.ts
index afadaff43e..b2ad98c627 100644
--- a/types/models.d.ts
+++ b/types/models.d.ts
@@ -1081,7 +1081,7 @@ declare module 'mongoose' {
     recompileSchema(): void;
 
     /** Schema the model uses. */
-    schema: Schema<TRawDocType>;
+    schema: TSchema;
 
     /** Creates a `updateMany` query: updates all documents that match `filter` with `update`. */
     updateMany<ResultDoc = THydratedDocumentType>(
diff --git a/types/utility.d.ts b/types/utility.d.ts
index 44d5ac1b0c..4c53706541 100644
--- a/types/utility.d.ts
+++ b/types/utility.d.ts
@@ -2,7 +2,7 @@ declare module 'mongoose' {
   type IfAny<IFTYPE, THENTYPE, ELSETYPE = IFTYPE> = 0 extends 1 & IFTYPE
     ? THENTYPE
     : ELSETYPE;
-  type IfUnknown<IFTYPE, THENTYPE> = unknown extends IFTYPE ? THENTYPE : IFTYPE;
+  type IsUnknown<T> = unknown extends T ? true : false;
 
   type WithLevel1NestedPaths<T, K extends keyof T = keyof T> = {
     [P in K | NestedPaths<Required<T>, K>]: P extends K
@@ -135,7 +135,7 @@ declare module 'mongoose' {
    */
   type AddThisParameter<T, D> = {
     [K in keyof T]: T[K] extends (...args: infer A) => infer R
-      ? ThisParameter<T[K], unknown> extends unknown
+      ? IsUnknown<ThisParameter<T[K], unknown>> extends true
         ? (this: D, ...args: A) => R
         : T[K]
       : T[K];