diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index 47b0d1e0..44735945 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -208,6 +208,13 @@ namespace WebSocketSharp
              : null;
     }
 
+    internal static string CheckIfCanClose (this WebSocketState state)
+    {
+      return state == WebSocketState.Closing || state == WebSocketState.Closed
+             ? String.Format ("This operation isn't available ({0}).", state)
+             : null;
+    }
+
     internal static string CheckIfCanConnect (this WebSocketState state)
     {
       return state == WebSocketState.Open || state == WebSocketState.Closing
@@ -224,15 +231,6 @@ namespace WebSocketSharp
                : null;
     }
 
-    internal static string CheckIfClosable (this WebSocketState state)
-    {
-      return state == WebSocketState.Closing
-             ? "While closing the WebSocket connection."
-             : state == WebSocketState.Closed
-               ? "The WebSocket connection has already been closed."
-               : null;
-    }
-
     internal static string CheckIfOpen (this WebSocketState state)
     {
       return state == WebSocketState.Connecting
diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index 0d7061be..933a99ee 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -1806,7 +1806,7 @@ namespace WebSocketSharp
     /// </summary>
     public void Close ()
     {
-      var msg = _readyState.CheckIfClosable ();
+      var msg = _readyState.CheckIfCanClose ();
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1830,7 +1830,7 @@ namespace WebSocketSharp
     /// </param>
     public void Close (ushort code)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, null, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, null, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1857,7 +1857,7 @@ namespace WebSocketSharp
     /// </param>
     public void Close (CloseStatusCode code)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, null, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, null, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1891,7 +1891,7 @@ namespace WebSocketSharp
     /// </param>
     public void Close (ushort code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, reason, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, reason, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1925,7 +1925,7 @@ namespace WebSocketSharp
     /// </param>
     public void Close (CloseStatusCode code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, reason, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, reason, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1950,7 +1950,7 @@ namespace WebSocketSharp
     /// </remarks>
     public void CloseAsync ()
     {
-      var msg = _readyState.CheckIfClosable ();
+      var msg = _readyState.CheckIfCanClose ();
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -1979,7 +1979,7 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (ushort code)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, null, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, null, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -2009,7 +2009,7 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (CloseStatusCode code)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, null, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, null, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -2048,7 +2048,7 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (ushort code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, reason, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, reason, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);
@@ -2088,7 +2088,7 @@ namespace WebSocketSharp
     /// </param>
     public void CloseAsync (CloseStatusCode code, string reason)
     {
-      var msg = _readyState.CheckIfClosable () ?? CheckCloseParameters (code, reason, _client);
+      var msg = _readyState.CheckIfCanClose () ?? CheckCloseParameters (code, reason, _client);
       if (msg != null) {
         _logger.Error (msg);
         error ("An error has occurred in closing the connection.", null);