diff --git a/testing/asserts.ts b/testing/asserts.ts
index 352bfd700735..94cca851bce2 100644
--- a/testing/asserts.ts
+++ b/testing/asserts.ts
@@ -161,6 +161,9 @@ export function equal(c: unknown, d: unknown): boolean {
         if (!compare(a && a[key as Key], b && b[key as Key])) {
           return false;
         }
+        if (((key in a) && (!(key in b))) || ((key in b) && (!(key in a)))) {
+          return false;
+        }
       }
       seen.set(a, b);
       return true;
diff --git a/testing/asserts_test.ts b/testing/asserts_test.ts
index f0eab61c6867..5ac83fce8c94 100644
--- a/testing/asserts_test.ts
+++ b/testing/asserts_test.ts
@@ -147,6 +147,12 @@ Deno.test("testingEqual", function (): void {
       new URL("https://example.test/with-path"),
     ),
   );
+  assert(
+    !equal({ a: undefined, b: undefined }, { a: undefined, c: undefined }),
+  );
+  assert(
+    !equal({ a: undefined, b: undefined }, { a: undefined }),
+  );
 });
 
 Deno.test("testingNotEquals", function (): void {