diff --git a/questions/00191-medium-append-argument/test-cases.ts b/questions/00191-medium-append-argument/test-cases.ts
index 229a0f11..8dcf57df 100644
--- a/questions/00191-medium-append-argument/test-cases.ts
+++ b/questions/00191-medium-append-argument/test-cases.ts
@@ -9,4 +9,6 @@ type Result2 = (x: undefined) => void
 type cases = [
   Expect<Equal<Case1, Result1>>,
   Expect<Equal<Case2, Result2>>,
+  // @ts-expect-error
+  AppendArgument<unknown, undefined>,
 ]