diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index b5ec9335..f73135ef 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -245,6 +245,13 @@ namespace WebSocketSharp
              : null;
     }
 
+    internal static string CheckIfCanStop (this ServerState state)
+    {
+      return state != ServerState.Start
+             ? String.Format ("This operation isn't available ({0}).", state)
+             : null;
+    }
+
     internal static string CheckIfStart (this ServerState state)
     {
       return state == ServerState.Ready
diff --git a/websocket-sharp/Server/HttpServer.cs b/websocket-sharp/Server/HttpServer.cs
index 06a01efb..01e3c3c7 100644
--- a/websocket-sharp/Server/HttpServer.cs
+++ b/websocket-sharp/Server/HttpServer.cs
@@ -911,7 +911,7 @@ namespace WebSocketSharp.Server
     public void Stop ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart ();
+        var msg = _state.CheckIfCanStop ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -939,7 +939,7 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -975,7 +975,7 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (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 93610dc6..cd9833ec 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -814,7 +814,7 @@ namespace WebSocketSharp.Server
     public void Stop ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart ();
+        var msg = _state.CheckIfCanStop ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -842,7 +842,7 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -877,7 +877,7 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfStart () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
         if (msg != null) {
           _logger.Error (msg);
           return;