diff --git a/yaml/_loader/loader.ts b/yaml/_loader/loader.ts
index c7260698c5ec..f0ffbd394d9c 100644
--- a/yaml/_loader/loader.ts
+++ b/yaml/_loader/loader.ts
@@ -300,7 +300,7 @@ function mergeMappings(
     );
   }
 
-  for (const key in Object.keys(source)) {
+  for (const key of Object.keys(source)) {
     if (!hasOwn(destination, key)) {
       Object.defineProperty(destination, key, {
         value: source[key],
diff --git a/yaml/parse_test.ts b/yaml/parse_test.ts
index adba7bd72688..33ce1e8ff676 100644
--- a/yaml/parse_test.ts
+++ b/yaml/parse_test.ts
@@ -448,3 +448,20 @@ Deno.test("parse() throws with invalid strings", () => {
     'expected valid JSON character at line 1, column 3:\n    "\b"\n      ^',
   );
 });
+
+Deno.test("parse() handles merge (<<) types", () => {
+  assertEquals(
+    parse(`<<: { a: 1, b: 2 }
+c: 3`),
+    { a: 1, b: 2, c: 3 },
+  );
+
+  assertThrows(
+    () =>
+      // number can't be used as merge value
+      parse(`<<: 1
+c: 3`),
+    YamlError,
+    "cannot merge mappings; the provided source object is unacceptable at line 1, column 6:\n    <<: 1\n         ^",
+  );
+});