diff --git a/index.d.ts b/index.d.ts
index 3184e817904..977c28a4452 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1704,8 +1704,9 @@ declare module "mongoose" {
 
   export type DocumentDefinition<T> = Omit<T, Exclude<keyof Document, '_id'>>;
 
-  type FunctionPropertyNames<T> = { 
-    [K in keyof T]: T[K] extends Function ? K : never 
+  type FunctionPropertyNames<T> = {
+    // The 1 & T[K] check comes from: https://stackoverflow.com/questions/55541275/typescript-check-for-the-any-type
+    [K in keyof T]: 0 extends (1 & T[K]) ? never : (T[K] extends Function ? K : never) 
   }[keyof T];
   export type LeanDocument<T> = Omit<Omit<T, Exclude<keyof Document, '_id'>>, FunctionPropertyNames<T>>;
 
diff --git a/test/typescript/leanDocuments.ts b/test/typescript/leanDocuments.ts
index 6c6dd5b2859..a05775138ab 100644
--- a/test/typescript/leanDocuments.ts
+++ b/test/typescript/leanDocuments.ts
@@ -5,6 +5,7 @@ const schema: Schema = new Schema({ name: { type: 'String' } });
 interface ITest extends Document {
   _id?: Types.ObjectId,
   name?: string;
+  mixed?: any;
   testMethod: () => number;
 }
 
@@ -24,6 +25,8 @@ void async function main() {
   await _doc.save();
 
   _doc.testMethod();
+  _doc.name = 'test';
+  _doc.mixed = 42;
 
   const hydrated = Test.hydrate(_doc);
   await hydrated.save();