diff --git a/.eslintignore b/.eslintignore
index c552bf922..74db35493 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -2,5 +2,6 @@ src/api/yorkie/v1/yorkie_grpc_web_pb.d.ts
 src/api/yorkie/v1/yorkie_pb.d.ts
 src/api/yorkie/v1/resources_grpc_web_pb.d.ts
 src/api/yorkie/v1/resources_pb.d.ts
+test/vitest.d.ts
 dist
 lib
diff --git a/src/api/converter.ts b/src/api/converter.ts
index d416a7fc2..75ca8310f 100644
--- a/src/api/converter.ts
+++ b/src/api/converter.ts
@@ -783,12 +783,16 @@ function toChangePack(pack: ChangePack<Indexable>): PbChangePack {
  * `errorCodeOf` returns the error code of the given connect error.
  */
 export function errorCodeOf(error: ConnectError): string {
+  // NOTE(hackerwins): Currently, we only use the first detail to represent the
+  // error code.
   const infos = error.findDetails(ErrorInfo);
   for (const info of infos) {
-    return info.metadata.code;
+    if (info.metadata.code) {
+      return info.metadata.code;
+    }
   }
 
-  return "";
+  return '';
 }
 
 /**
diff --git a/test/integration/client_test.ts b/test/integration/client_test.ts
index 2e3c8c5d6..7477f7dc4 100644
--- a/test/integration/client_test.ts
+++ b/test/integration/client_test.ts
@@ -1,3 +1,19 @@
+/*
+ * Copyright 2024 The Yorkie Authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 import { describe, it, assert, vi, afterEach, expect } from 'vitest';
 
 import yorkie, {
@@ -15,7 +31,8 @@ import {
   testRPCAddr,
   withTwoClientsAndDocuments,
 } from '@yorkie-js-sdk/test/integration/integration_helper';
-import { ConnectError, Code } from '@connectrpc/connect';
+import { ConnectError, Code as ConnectCode } from '@connectrpc/connect';
+import { Code } from '@yorkie-js-sdk/src/util/error';
 
 describe.sequential('Client', function () {
   afterEach(() => {
@@ -49,12 +66,22 @@ describe.sequential('Client', function () {
   it('Can attach/detach document', async function ({ task }) {
     const cli = new yorkie.Client(testRPCAddr);
     await cli.activate();
-
-    const key = toDocKey(`${task.name}-${new Date().getTime()}`);
-    const doc = new yorkie.Document<{ version: number }>(key);
+    const doc = new yorkie.Document(
+      toDocKey(`${task.name}-${new Date().getTime()}`),
+    );
 
     await cli.attach(doc);
-    await expect(async () => cli.attach(doc)).rejects.toThrow('is not detached');
+    expect(async () => cli.attach(doc)).rejects.toThrowErrorCode(
+      Code.DocumentNotDetached,
+    );
+
+    await cli.detach(doc);
+    expect(async () => cli.detach(doc)).rejects.toThrowErrorCode(
+      Code.DocumentNotAttached,
+    );
+
+    await cli.deactivate();
+    await cli.deactivate();
   });
 
   it('Can handle sync', async function ({ task }) {
@@ -790,7 +817,7 @@ describe.sequential('Client', function () {
 
     // 01. Simulate Unknown error.
     vi.stubGlobal('fetch', async () => {
-      throw new ConnectError('Failed to fetch', Code.Unknown);
+      throw new ConnectError('Failed to fetch', ConnectCode.Unknown);
     });
 
     doc.update((root) => {
@@ -803,7 +830,7 @@ describe.sequential('Client', function () {
 
     // 02. Simulate FailedPrecondition error.
     vi.stubGlobal('fetch', async () => {
-      throw new ConnectError('Failed to fetch', Code.FailedPrecondition);
+      throw new ConnectError('Failed to fetch', ConnectCode.FailedPrecondition);
     });
 
     await new Promise((res) => setTimeout(res, 30));
diff --git a/test/vitest.d.ts b/test/vitest.d.ts
new file mode 100644
index 000000000..16f97b455
--- /dev/null
+++ b/test/vitest.d.ts
@@ -0,0 +1,27 @@
+/*
+ * Copyright 2024 The Yorkie Authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import type { Assertion, AsymmetricMatchersContaining } from 'vitest';
+import { Code } from '@yorkie-js-sdk/src/util/error';
+
+interface CustomMatchers<R = unknown> {
+  toThrowErrorCode: (expected: Code) => R;
+}
+
+declare module 'vitest' {
+  type Assertion<T = any> = CustomMatchers<T>;
+  interface AsymmetricMatchersContaining extends CustomMatchers {}
+}
diff --git a/test/vitest.setup.ts b/test/vitest.setup.ts
new file mode 100644
index 000000000..86a6848df
--- /dev/null
+++ b/test/vitest.setup.ts
@@ -0,0 +1,29 @@
+/*
+ * Copyright 2024 The Yorkie Authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { expect } from 'vitest';
+import { Code } from '@yorkie-js-sdk/src/util/error';
+
+expect.extend({
+  toThrowErrorCode(received: any, expected: Code) {
+    const { isNot } = this;
+    const pass = received?.code === expected;
+    return {
+      pass,
+      message: () => `${received} is${isNot ? ' not' : ''} expected`,
+    };
+  },
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index 2bf9616f4..e2afb57ae 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -20,6 +20,7 @@ export default defineConfig({
     benchmark: {
       exclude: ['**/lib/**', '**/node_modules/**'],
     },
+    setupFiles: ['./test/vitest.setup.ts'],
   },
   plugins: [
     tsconfigPaths({