diff --git a/packages/core/test/signatures.test.ts b/packages/core/test/signatures.test.ts
index f11e0e9..942e674 100644
--- a/packages/core/test/signatures.test.ts
+++ b/packages/core/test/signatures.test.ts
@@ -13,7 +13,7 @@ describe('协议签名原语', () => {
     const signature = h(fixture.signature);
     expect(await verifyStatement(publicKey, fixture.kind, payload, signature)).toBe(true);
     expect(await verifyStatement(publicKey, 'chat.edit', payload, signature)).toBe(false);
-    payload[0] ^= 1;
+    payload[0] = payload[0]! ^ 1;
     expect(await verifyStatement(publicKey, fixture.kind, payload, signature)).toBe(false);
     expect(await verifyStatement(publicKey, fixture.kind, h(fixture.payload), signature.subarray(1))).toBe(false);
     expect(fromBase64Url(fixture.x)).toHaveLength(32);