diff --git a/src/stringify/stringifyPair.ts b/src/stringify/stringifyPair.ts
index ad9fabcd..4ef6710f 100644
--- a/src/stringify/stringifyPair.ts
+++ b/src/stringify/stringifyPair.ts
@@ -22,7 +22,7 @@ export function stringifyPair(
     if (keyComment) {
       throw new Error('With simple keys, key nodes cannot have comments')
     }
-    if (isCollection(key)) {
+    if (isCollection(key) || (!isNode(key) && typeof key === 'object')) {
       const msg = 'With simple keys, collection cannot be used as a key value'
       throw new Error(msg)
     }
diff --git a/tests/doc/stringify.ts b/tests/doc/stringify.ts
index 0ff20d58..7a150e63 100644
--- a/tests/doc/stringify.ts
+++ b/tests/doc/stringify.ts
@@ -723,6 +723,24 @@ describe('simple keys', () => {
     )
   })
 
+  test('key with JS object value', () => {
+    const doc = YAML.parseDocument<any>('[foo]: bar')
+    doc.contents.items[0].key = { foo: 42 }
+    expect(doc.toString()).toBe('? foo: 42\n: bar\n')
+    expect(() => doc.toString({ simpleKeys: true })).toThrow(
+      /With simple keys, collection cannot be used as a key value/
+    )
+  })
+
+  test('key with JS null value', () => {
+    const doc = YAML.parseDocument<any>('[foo]: bar')
+    doc.contents.items[0].key = null
+    expect(doc.toString()).toBe('? null\n: bar\n')
+    expect(() => doc.toString({ simpleKeys: true })).toThrow(
+      /With simple keys, collection cannot be used as a key value/
+    )
+  })
+
   test('key value lingth > 1024', () => {
     const str = `
     ? ${new Array(1026).join('a')}