diff --git a/src/JSSoft.Communication/Grpc/AdaptorClient.cs b/src/JSSoft.Communication/Grpc/AdaptorClient.cs
index 6cf7899..9de71e1 100644
--- a/src/JSSoft.Communication/Grpc/AdaptorClient.cs
+++ b/src/JSSoft.Communication/Grpc/AdaptorClient.cs
@@ -403,23 +403,31 @@ private void InvokeCallback(IService service, string name, string[] data)
         }
 
         var methodDescriptors = _methodsByService[service];
-        if (methodDescriptors.Contains(name) != true)
+        if (methodDescriptors.Contains(name) == true)
         {
-            throw new InvalidOperationException("Invalid method name.");
+            var methodDescriptor = methodDescriptors[name];
+            var args = _serializer!.DeserializeMany(methodDescriptor.ParameterTypes, data);
+            var instance = _descriptor!.ClientInstances[service];
+            Task.Run(() => methodDescriptor.InvokeAsync(_serviceContext, instance, args));
+        }
+        else
+        {
+            LogUtility.Warn($"Method '{name}' is not found.");
         }
-
-        var methodDescriptor = methodDescriptors[name];
-        var args = _serializer!.DeserializeMany(methodDescriptor.ParameterTypes, data);
-        var instance = _descriptor!.ClientInstances[service];
-        Task.Run(() => methodDescriptor.InvokeAsync(_serviceContext, instance, args));
     }
 
     private void InvokeCallback(PollReply reply)
     {
         foreach (var item in reply.Items)
         {
-            var service = _serviceByName[item.ServiceName];
-            InvokeCallback(service, item.Name, [.. item.Data]);
+            if (_serviceByName.TryGetValue(item.ServiceName, out var service) == true)
+            {
+                InvokeCallback(service, item.Name, [.. item.Data]);
+            }
+            else
+            {
+                LogUtility.Warn($"Service '{item.ServiceName}' is not found.");
+            }
         }
 
         reply.Items.Clear();
diff --git a/test/JSSoft.Communication.Tests/CallbackNoneTest.cs b/test/JSSoft.Communication.Tests/CallbackNoneTest.cs
new file mode 100644
index 0000000..7e63738
--- /dev/null
+++ b/test/JSSoft.Communication.Tests/CallbackNoneTest.cs
@@ -0,0 +1,83 @@
+// <copyright file="CallbackNoneTest.cs" company="JSSoft">
+//   Copyright (c) 2024 Jeesu Choi. All Rights Reserved.
+//   Licensed under the MIT License. See LICENSE.md in the project root for license information.
+// </copyright>
+
+using JSSoft.Communication.Tests.Extensions;
+using Xunit.Abstractions;
+
+namespace JSSoft.Communication.Tests;
+
+public class CallbackNoneTest : IAsyncLifetime
+{
+    private const int Timeout = 3000;
+    private readonly ITestOutputHelper _logger;
+    private readonly TestServer1 _testServer = new();
+    private readonly ServerContext _serverContext;
+    private readonly ClientContext _clientContext;
+    private readonly RandomEndPoint _endPoint = new();
+    private ITestService1? _server;
+
+    private Guid _clientToken;
+    private Guid _serverToken;
+
+    public CallbackNoneTest(ITestOutputHelper logger)
+    {
+        _logger = logger;
+        _serverContext = new(_testServer) { EndPoint = _endPoint };
+        _clientContext = new(new ClientService<ITestService1>()) { EndPoint = _endPoint };
+        logger.WriteLine($"{_endPoint}");
+    }
+
+    public interface ITestService1
+    {
+        void Invoke();
+    }
+
+    public interface ITestService2
+    {
+        void Invoke();
+    }
+
+    public interface ITestCallback2
+    {
+        void OnInvoked();
+    }
+
+    [Fact]
+    public void Callback1_Test()
+    {
+        var manualResetEvent = new ManualResetEvent(false);
+        _clientContext.Disconnected += ClientContext_Disconnected;
+        _server!.Invoke();
+        Assert.False(manualResetEvent.WaitOne(Timeout));
+
+        void ClientContext_Disconnected(object? sender, EventArgs e)
+        {
+            manualResetEvent.Set();
+        }
+    }
+
+    public async Task InitializeAsync()
+    {
+        _serverToken = await _serverContext.OpenAsync(CancellationToken.None);
+        _logger.WriteLine($"Server is opened: {_serverToken}");
+        _clientToken = await _clientContext.OpenAsync(CancellationToken.None);
+        _logger.WriteLine($"Client is opened: {_clientToken}");
+        _server = _testServer;
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _serverContext.ReleaseAsync(_serverToken);
+        _logger.WriteLine($"Server is released: {_serverToken}");
+        await _clientContext.ReleaseAsync(_clientToken);
+        _logger.WriteLine($"Client is released: {_clientToken}");
+        _endPoint.Dispose();
+    }
+
+    private sealed class TestServer1 : ServerService<ITestService1, ITestCallback2>, ITestService1
+    {
+        public void Invoke() => Client.OnInvoked();
+    }
+}
diff --git a/test/JSSoft.Communication.Tests/CallbackTest.cs b/test/JSSoft.Communication.Tests/CallbackTest.cs
index 02670a5..42c73a2 100644
--- a/test/JSSoft.Communication.Tests/CallbackTest.cs
+++ b/test/JSSoft.Communication.Tests/CallbackTest.cs
@@ -10,23 +10,28 @@ namespace JSSoft.Communication.Tests;
 
 public class CallbackTest : IAsyncLifetime
 {
-    private const int Timeout = 30000;
+    private const int ClientCount = 2;
     private readonly ITestOutputHelper _logger;
     private readonly TestServer _testServer = new();
-    private readonly TestClient _testClient = new();
     private readonly ServerContext _serverContext;
-    private readonly ClientContext _clientContext;
+    private readonly TestClient[] _testClients = new TestClient[ClientCount];
+    private readonly ClientContext[] _clientContexts = new ClientContext[ClientCount];
+    private readonly Guid[] _clientTokens = new Guid[ClientCount];
     private readonly RandomEndPoint _endPoint = new();
-    private ITestService? _server;
+    private TestServer? _server;
 
-    private Guid _clientToken;
     private Guid _serverToken;
 
     public CallbackTest(ITestOutputHelper logger)
     {
         _logger = logger;
         _serverContext = new(_testServer) { EndPoint = _endPoint };
-        _clientContext = new(_testClient) { EndPoint = _endPoint };
+        for (var i = 0; i < ClientCount; i++)
+        {
+            _testClients[i] = new() { Index = i };
+            _clientContexts[i] = new(_testClients[i]) { EndPoint = _endPoint };
+        }
+
         logger.WriteLine($"{_endPoint}");
     }
 
@@ -49,55 +54,63 @@ public interface ITestCallback
     }
 
     [Fact]
-    public void Callback1_Test()
+    public async Task Callback1_TestAsync()
     {
-        var raised = Assert.Raises<ValueEventArgs>(
-            handler => _testClient.Invoked += handler,
-            handler => _testClient.Invoked -= handler,
-            () =>
+        var raisedCount = await EventTestUtility.RaisesManyAsync<TestClient, ValueEventArgs>(
+            items: _testClients,
+            attach: (item, handler) => item.Invoked += handler,
+            detach: (item, handler) => item.Invoked -= handler,
+            testCode: () =>
             {
                 _server!.Invoke();
-                _testClient.AutoResetEvent.WaitOne(Timeout);
             });
-        Assert.Null(raised.Arguments.Value);
+
+        Assert.Equal(_testClients.Length, raisedCount);
+        Assert.All(_testClients, item => Assert.Null(item.Value));
     }
 
     [Fact]
-    public void Callback2_Test()
+    public async Task Callback2_TestAsync()
     {
         var value = 123;
-        var raised = Assert.Raises<ValueEventArgs>(
-            handler => _testClient.Invoked += handler,
-            handler => _testClient.Invoked -= handler,
-            () =>
+        var raisedCount = await EventTestUtility.RaisesManyAsync<TestClient, ValueEventArgs>(
+            items: _testClients,
+            attach: (item, handler) => item.Invoked += handler,
+            detach: (item, handler) => item.Invoked -= handler,
+            testCode: () =>
             {
                 _server!.Invoke(value);
-                _testClient.AutoResetEvent.WaitOne(Timeout);
             });
-        Assert.Equal(value, raised.Arguments.Value);
+        Assert.Equal(_testClients.Length, raisedCount);
+        Assert.All(_testClients, item => Assert.Equal(value, item.Value));
     }
 
     [Fact]
-    public void Callback3_Test()
+    public async Task Callback3_TestAsync()
     {
         var value = (123, "123");
-        var raised = Assert.Raises<ValueEventArgs>(
-            handler => _testClient.Invoked += handler,
-            handler => _testClient.Invoked -= handler,
-            () =>
+        var raisedCount = await EventTestUtility.RaisesManyAsync<TestClient, ValueEventArgs>(
+            items: _testClients,
+            attach: (item, handler) => item.Invoked += handler,
+            detach: (item, handler) => item.Invoked -= handler,
+            testCode: () =>
             {
                 _server!.Invoke(value);
-                _testClient.AutoResetEvent.WaitOne(Timeout);
             });
-        Assert.Equal(value, raised.Arguments.Value);
+        Assert.Equal(_testClients.Length, raisedCount);
+        Assert.All(_testClients, item => Assert.Equal(value, item.Value));
     }
 
     public async Task InitializeAsync()
     {
         _serverToken = await _serverContext.OpenAsync(CancellationToken.None);
         _logger.WriteLine($"Server is opened: {_serverToken}");
-        _clientToken = await _clientContext.OpenAsync(CancellationToken.None);
-        _logger.WriteLine($"Client is opened: {_clientToken}");
+        for (var i = 0; i < ClientCount; i++)
+        {
+            _clientTokens[i] = await _clientContexts[i].OpenAsync(CancellationToken.None);
+            _logger.WriteLine($"Client #{i} is opened: {_clientTokens[i]}");
+        }
+
         _server = _testServer;
     }
 
@@ -105,8 +118,12 @@ public async Task DisposeAsync()
     {
         await _serverContext.ReleaseAsync(_serverToken);
         _logger.WriteLine($"Server is released: {_serverToken}");
-        await _clientContext.ReleaseAsync(_clientToken);
-        _logger.WriteLine($"Client is released: {_clientToken}");
+        for (var i = 0; i < ClientCount; i++)
+        {
+            await _clientContexts[i].ReleaseAsync(_clientTokens[i]);
+            _logger.WriteLine($"Client #{i} is released: {_clientTokens[i]}");
+        }
+
         _endPoint.Dispose();
     }
 
@@ -130,20 +147,27 @@ private sealed class TestClient : ClientService<ITestService, ITestCallback>, IT
 
         public AutoResetEvent AutoResetEvent { get; } = new(initialState: false);
 
+        public int Index { get; set; } = -1;
+
+        public object? Value { get; private set; } = DBNull.Value;
+
         void ITestCallback.OnInvoked()
         {
+            Value = null;
             Invoked?.Invoke(this, new(null));
             AutoResetEvent.Set();
         }
 
         void ITestCallback.OnInvoked(int value)
         {
+            Value = value;
             Invoked?.Invoke(this, new(value));
             AutoResetEvent.Set();
         }
 
         void ITestCallback.OnInvoked((int Value1, string Value2) value)
         {
+            Value = value;
             Invoked?.Invoke(this, new(value));
             AutoResetEvent.Set();
         }
diff --git a/test/JSSoft.Communication.Tests/EventObjectCollection.cs b/test/JSSoft.Communication.Tests/EventObjectCollection.cs
new file mode 100644
index 0000000..0e513b9
--- /dev/null
+++ b/test/JSSoft.Communication.Tests/EventObjectCollection.cs
@@ -0,0 +1,53 @@
+// <copyright file="EventObjectCollection.cs" company="JSSoft">
+//   Copyright (c) 2024 Jeesu Choi. All Rights Reserved.
+//   Licensed under the MIT License. See LICENSE.md in the project root for license information.
+// </copyright>
+
+namespace JSSoft.Communication.Tests;
+
+public sealed class EventObjectCollection<TObject, TEventArgs>
+    : Dictionary<TObject, ManualResetEvent>, IDisposable
+    where TObject : notnull
+    where TEventArgs : EventArgs
+{
+    private readonly Action<TObject, EventHandler<TEventArgs>> _detach;
+
+    public EventObjectCollection(
+        TObject[] items,
+        Action<TObject, EventHandler<TEventArgs>> attach,
+        Action<TObject, EventHandler<TEventArgs>> detach)
+        : base(capacity: items.Length)
+    {
+        for (var i = 0; i < items.Length; i++)
+        {
+            Add(items[i], new ManualResetEvent(initialState: false));
+            attach(items[i], Handler);
+        }
+
+        _detach = detach;
+    }
+
+    public void Dispose()
+    {
+        foreach (var item in this)
+        {
+            _detach(item.Key, Handler);
+            item.Value.Dispose();
+        }
+    }
+
+    public async Task<int> WaiyAsync(int timeout)
+    {
+        var tasks = Values.Select(item => Task.Run(() => item.WaitOne(timeout)));
+        var results = await Task.WhenAll(tasks);
+        return results.Count(item => item == true);
+    }
+
+    private void Handler(object? s, TEventArgs args)
+    {
+        if (s is TObject item && TryGetValue(item, out var manualResetEvent) == true)
+        {
+            manualResetEvent.Set();
+        }
+    }
+}
diff --git a/test/JSSoft.Communication.Tests/EventTestUtility.cs b/test/JSSoft.Communication.Tests/EventTestUtility.cs
index 9232995..a630e25 100644
--- a/test/JSSoft.Communication.Tests/EventTestUtility.cs
+++ b/test/JSSoft.Communication.Tests/EventTestUtility.cs
@@ -7,6 +7,8 @@ namespace JSSoft.Communication.Tests;
 
 public static class EventTestUtility
 {
+    public const int Timeout = 10000;
+
     public static async Task<bool> RaisesAsync(
         Action<EventHandler> attach, Action<EventHandler> detach, Func<Task> testCode)
     {
@@ -25,11 +27,39 @@ public static async Task<bool> RaisesAsync(
             detach(Handler);
         }
 
-        return manualResetEvent.WaitOne(millisecondsTimeout: 10000);
+        return manualResetEvent.WaitOne(Timeout);
 
         void Handler(object? s, EventArgs args)
         {
             manualResetEvent.Set();
         }
     }
+
+    public static async Task<int> RaisesManyAsync<TObject, TEventArgs>(
+        TObject[] items,
+        Action<TObject, EventHandler<TEventArgs>> attach,
+        Action<TObject, EventHandler<TEventArgs>> detach,
+        Action testCode)
+        where TObject : notnull
+        where TEventArgs : EventArgs
+    {
+        using var eventByItem
+            = new EventObjectCollection<TObject, TEventArgs>(items, attach, detach);
+        testCode();
+        return await eventByItem.WaiyAsync(Timeout);
+    }
+
+    public static async Task<int> RaisesManyAsync<TObject, TEventArgs>(
+        TObject[] items,
+        Action<TObject, EventHandler<TEventArgs>> attach,
+        Action<TObject, EventHandler<TEventArgs>> detach,
+        Func<Task> testCode)
+        where TObject : notnull
+        where TEventArgs : EventArgs
+    {
+        using var eventByItem
+            = new EventObjectCollection<TObject, TEventArgs>(items, attach, detach);
+        await testCode();
+        return await eventByItem.WaiyAsync(Timeout);
+    }
 }