diff --git a/src/foundry/common/utils/collection.d.mts b/src/foundry/common/utils/collection.d.mts
index 8d4181aa6..a2cfa55d0 100644
--- a/src/foundry/common/utils/collection.d.mts
+++ b/src/foundry/common/utils/collection.d.mts
@@ -1,31 +1,17 @@
-interface MapReplacementMembers<K, V> {
-  set(key: K, value: V): this;
+// This class exists make it as sound as possible to override these parts of the class and make them
+// completely unrelated. It's done this way specifically to avoid situations like
+declare class LenientMap<K, V> extends Map<K, V> {
+  [Symbol.iterator](): any;
+  forEach(...args: any[]): any;
 }
 
-type PatchedMap<K, V> = Omit<Map<K, V>, "forEach" | typeof Symbol.iterator | "get" | "set"> &
-  MapReplacementMembers<K, V>;
-
-declare namespace PatchedMap {
-  type Any = PatchedMap<any, any>;
-}
-
-interface PatchedMapConstructor {
-  new (): PatchedMap.Any;
-  new <K, V>(entries?: readonly (readonly [K, V])[] | null): PatchedMap<K, V>;
-  new <K, V>(iterable: Iterable<readonly [K, V]>): PatchedMap<K, V>;
-  readonly [Symbol.species]: PatchedMapConstructor;
-  readonly prototype: PatchedMap.Any;
-}
-
-declare const Map: PatchedMapConstructor;
-
 /**
  * A reusable storage concept which blends the functionality of an Array with the efficient key-based lookup of a Map.
  * This concept is reused throughout Foundry VTT where a collection of uniquely identified elements is required.
  * @typeParam T - The type of the objects contained in the Collection
  */
-declare class Collection<V> extends Map<string, V> {
-  constructor(entries?: readonly (readonly [string, V])[] | null);
+declare class Collection<V> extends LenientMap<string, V> {
+  constructor(entries?: Iterable<readonly [string, V]> | null);
 
   /**
    * When iterating over a Collection, we should iterate over its values instead of over its entries
diff --git a/tests/foundry/common/utils/collection.mjs.test-d.ts b/tests/foundry/common/utils/collection.mjs.test-d.ts
index 8733b3bbd..1511cb2c0 100644
--- a/tests/foundry/common/utils/collection.mjs.test-d.ts
+++ b/tests/foundry/common/utils/collection.mjs.test-d.ts
@@ -15,8 +15,25 @@ function isString(e: string | null): e is string {
 }
 
 const cn = new Collection<string | null>();
-expectTypeOf(cn.filter((each) => typeof each === "string")).toEqualTypeOf<Array<string | null>>();
-expectTypeOf(cn.find((each) => typeof each === "string")).toEqualTypeOf<string | null | undefined>();
+expectTypeOf(cn.filter((each) => typeof each === "string")).toEqualTypeOf<Array<string>>();
+expectTypeOf(cn.find((each) => typeof each === "string")).toEqualTypeOf<string | undefined>();
 
 expectTypeOf(cn.filter(isString)).toEqualTypeOf<string[]>();
 expectTypeOf(cn.find(isString)).toEqualTypeOf<string | undefined>();
+
+// This is a regression test for the error:
+//    Class '...' defines instance member property '...', but extended class '...' defines it as instance member function.
+// This occurred because of how `Map` was patched to allow the unsound subclassing of `Collection`.
+class CustomCollection extends Collection<string> {
+  override clear(): void {}
+
+  override delete(_key: string): boolean {
+    return true;
+  }
+}
+
+declare const customCollection: CustomCollection;
+
+if (customCollection instanceof Map) {
+  expectTypeOf(customCollection).toEqualTypeOf<CustomCollection>();
+}