diff --git a/packages/cli/src/utils/motionSpec.test.ts b/packages/cli/src/utils/motionSpec.test.ts
index c336cfe6bb3..3c867fe46f4 100644
--- a/packages/cli/src/utils/motionSpec.test.ts
+++ b/packages/cli/src/utils/motionSpec.test.ts
@@ -47,6 +47,16 @@ describe("parseMotionSpec", () => {
     if (!result.ok) expect(result.errors[0]).toContain("unknown assertion kind");
   });
 
+  it.each(["constructor", "__proto__", "hasOwnProperty", "toString", "valueOf"])(
+    "rejects inherited validator name %s as an unknown assertion kind",
+    (kind) => {
+      expect(parseMotionSpec({ assertions: [{ kind }] })).toEqual({
+        ok: false,
+        errors: [`assertions[0]: unknown assertion kind ${JSON.stringify(kind)}`],
+      });
+    },
+  );
+
   it("reports per-field errors for missing required fields", () => {
     const result = parseMotionSpec({
       assertions: [
@@ -160,6 +170,22 @@ describe("readMotionSpec", () => {
     if (!result.ok) expect(result.errors[0]).toContain("no assertions");
   });
 
+  it("reports an inherited kind alongside other validation errors from a sidecar", () => {
+    const dir = tempDir("motion-inherited-");
+    const path = join(dir, "index.motion.json");
+    writeFileSync(
+      path,
+      JSON.stringify({ assertions: [{ kind: "__proto__" }, { kind: "before", a: "#a" }] }),
+    );
+    expect(readMotionSpec(path)).toEqual({
+      ok: false,
+      errors: [
+        'assertions[0]: unknown assertion kind "__proto__"',
+        'assertions[1] (before): "b" must be a non-empty string',
+      ],
+    });
+  });
+
   it("parses a valid sidecar file", () => {
     const dir = tempDir("motion-valid-");
     const path = join(dir, "main.motion.json");
diff --git a/packages/cli/src/utils/motionSpec.ts b/packages/cli/src/utils/motionSpec.ts
index b6dec6bab3b..dd0440f653e 100644
--- a/packages/cli/src/utils/motionSpec.ts
+++ b/packages/cli/src/utils/motionSpec.ts
@@ -69,7 +69,10 @@ const VALIDATORS: Record<string, Validator> = {
 function validateAssertion(raw: unknown, index: number): MotionAssertion | string {
   const at = `assertions[${index}]`;
   if (!isObject(raw)) return `${at}: must be an object`;
-  const validator = typeof raw.kind === "string" ? VALIDATORS[raw.kind] : undefined;
+  const validator =
+    typeof raw.kind === "string" && Object.hasOwn(VALIDATORS, raw.kind)
+      ? VALIDATORS[raw.kind]
+      : undefined;
   if (!validator) return `${at}: unknown assertion kind ${JSON.stringify(raw.kind)}`;
   return validator(raw, at);
 }