diff --git a/src/fake/fake_mixin.ts b/src/fake/fake_mixin.ts
index bdc9552..05d06f7 100644
--- a/src/fake/fake_mixin.ts
+++ b/src/fake/fake_mixin.ts
@@ -79,7 +79,7 @@ export function createFake<OriginalConstructor, OriginalObject>(
         methodName,
       );
 
-      if (!((methodName as string) in this.#original)) {
+      if (!((methodName as string) in (this.#original as Record<string, unknown>))) {
         const typeSafeMethodName = String(methodName as string);
 
         throw new FakeError(
diff --git a/src/mock/mock_mixin.ts b/src/mock/mock_mixin.ts
index fb84cc7..fb8f716 100644
--- a/src/mock/mock_mixin.ts
+++ b/src/mock/mock_mixin.ts
@@ -196,7 +196,7 @@ export function createMock<OriginalConstructor, OriginalObject>(
         methodName,
       );
 
-      if (!((methodName as string) in this.#original)) {
+      if (!((methodName as string) in (this.#original as Record<string, unknown>))) {
         const typeSafeMethodName = String(methodName);
         throw new MockError(
           `Method "${typeSafeMethodName}" does not exist.`,