diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index e1cb9606..45a966e4 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -251,33 +251,6 @@ namespace WebSocketSharp
                : null;
     }
 
-    internal static string CheckIfValidCloseParameters (this ushort code, string reason)
-    {
-      return !code.IsCloseStatusCode ()
-             ? "An invalid close status code."
-             : code.IsNoStatus () && !reason.IsNullOrEmpty ()
-               ? "NoStatus cannot have a reason."
-               : !reason.IsNullOrEmpty () && Encoding.UTF8.GetBytes (reason).Length > 123
-                 ? "A reason has greater than the allowable max size."
-                 : null;
-    }
-
-    internal static string CheckIfValidCloseParameters (this CloseStatusCode code, string reason)
-    {
-      return code.IsNoStatus () && !reason.IsNullOrEmpty ()
-             ? "NoStatus cannot have a reason."
-             : !reason.IsNullOrEmpty () && Encoding.UTF8.GetBytes (reason).Length > 123
-               ? "A reason has greater than the allowable max size."
-               : null;
-    }
-
-    internal static string CheckIfValidCloseStatusCode (this ushort code)
-    {
-      return !code.IsCloseStatusCode ()
-             ? "An invalid close status code."
-             : null;
-    }
-
     internal static string CheckIfValidControlData (this byte[] data, string paramName)
     {
       return data.Length > 125
diff --git a/websocket-sharp/Server/HttpServer.cs b/websocket-sharp/Server/HttpServer.cs
index 97ce3d11..222b30d3 100644
--- a/websocket-sharp/Server/HttpServer.cs
+++ b/websocket-sharp/Server/HttpServer.cs
@@ -757,7 +757,9 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? code.CheckIfValidCloseParameters (reason);
+        var msg = _state.CheckIfStart () ??
+                  WebSocket.CheckIfValidCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -793,7 +795,9 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? code.CheckIfValidCloseParameters (reason);
+        var msg = _state.CheckIfStart () ??
+                  WebSocket.CheckIfValidCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/Server/WebSocketServer.cs b/websocket-sharp/Server/WebSocketServer.cs
index 7ba2a52d..f6159b6d 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -842,7 +842,9 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? code.CheckIfValidCloseParameters (reason);
+        var msg = _state.CheckIfStart () ??
+                  WebSocket.CheckIfValidCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -877,7 +879,9 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? code.CheckIfValidCloseParameters (reason);
+        var msg = _state.CheckIfStart () ??
+                  WebSocket.CheckIfValidCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index eb2fca4b..5b6964b2 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -1560,6 +1560,35 @@ namespace WebSocketSharp
 
     #region Internal Methods
 
+    internal static string CheckIfValidCloseParameters (ushort code, string reason, bool client)
+    {
+      return !code.IsCloseStatusCode ()
+             ? "An invalid close status code."
+             : code == (ushort) CloseStatusCode.NoStatus
+               ? (!reason.IsNullOrEmpty () ? "NoStatus cannot have a reason." : null)
+               : code == (ushort) CloseStatusCode.MandatoryExtension && !client
+                 ? "MandatoryExtension cannot be used by the server."
+                 : code == (ushort) CloseStatusCode.ServerError && client
+                   ? "ServerError cannot be used by the client."
+                   : !reason.IsNullOrEmpty () && Encoding.UTF8.GetBytes (reason).Length > 123
+                     ? "A reason has greater than the allowable max size."
+                     : null;
+    }
+
+    internal static string CheckIfValidCloseParameters (
+      CloseStatusCode code, string reason, bool client)
+    {
+      return code == CloseStatusCode.NoStatus
+             ? (!reason.IsNullOrEmpty () ? "NoStatus cannot have a reason." : null)
+             : code == CloseStatusCode.MandatoryExtension && !client
+               ? "MandatoryExtension cannot be used by the server."
+               : code == CloseStatusCode.ServerError && client
+                 ? "ServerError cannot be used by the client."
+                 : !reason.IsNullOrEmpty () && Encoding.UTF8.GetBytes (reason).Length > 123
+                   ? "A reason has greater than the allowable max size."
+                   : null;
+    }
+
     // As server
     internal void Close (HttpResponse response)
     {
@@ -1738,7 +1767,9 @@ namespace WebSocketSharp
     /// </param>
     public void Close (ushort code)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseStatusCode ();
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, null, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1765,7 +1796,9 @@ namespace WebSocketSharp
     /// </param>
     public void Close (CloseStatusCode code)
     {
-      var msg = _readyState.CheckIfClosable ();
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, null, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1799,7 +1832,9 @@ namespace WebSocketSharp
     /// </param>
     public void Close (ushort code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseParameters (reason);
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, reason, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1833,7 +1868,9 @@ namespace WebSocketSharp
     /// </param>
     public void Close (CloseStatusCode code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseParameters (reason);
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, reason, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1887,7 +1924,9 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (ushort code)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseStatusCode ();
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, null, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1917,7 +1956,9 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (CloseStatusCode code)
     {
-      var msg = _readyState.CheckIfClosable ();
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, null, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1956,7 +1997,9 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (ushort code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseParameters (reason);
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, reason, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1996,7 +2039,9 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (CloseStatusCode code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? code.CheckIfValidCloseParameters (reason);
+      var msg = _readyState.CheckIfClosable () ??
+                CheckIfValidCloseParameters (code, reason, _client);
+
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);