Fix isPlainObject

colinhacks · colinhacks · commit 33495d51f2c5 · 2025-05-28T17:29:12.000-07:00
diff --git a/packages/zod/src/v4/classic/tests/index.test.ts b/packages/zod/src/v4/classic/tests/index.test.ts
@@ -775,3 +775,15 @@ test("z.promise", async () => {
 //   // @ts-expect-error
 //   schema.assertOutput<string>();
 // });
+
+test("isPlainObject", () => {
+  expect(z.core.util.isPlainObject({})).toEqual(true);
+  expect(z.core.util.isPlainObject(Object.create(null))).toEqual(true);
+  expect(z.core.util.isPlainObject([])).toEqual(false);
+  expect(z.core.util.isPlainObject(new Date())).toEqual(false);
+  expect(z.core.util.isPlainObject(null)).toEqual(false);
+  expect(z.core.util.isPlainObject(undefined)).toEqual(false);
+  expect(z.core.util.isPlainObject("string")).toEqual(false);
+  expect(z.core.util.isPlainObject(123)).toEqual(false);
+  expect(z.core.util.isPlainObject(Symbol())).toEqual(false);
+});
diff --git a/packages/zod/src/v4/core/util.ts b/packages/zod/src/v4/core/util.ts
@@ -333,7 +333,11 @@ export const allowsEval: { value: boolean } = cached(() => {
 });
 
 export function isPlainObject(data: any): data is Record<PropertyKey, unknown> {
-  return typeof data === "object" && data !== null && Object.getPrototypeOf(data) === Object.prototype;
+  return (
+    typeof data === "object" &&
+    data !== null &&
+    (Object.getPrototypeOf(data) === Object.prototype || Object.getPrototypeOf(data) === null)
+  );
 }
 
 export function numKeys(data: any): number {
