diff --git a/websocket-sharp/Server/WebSocketBehavior.cs b/websocket-sharp/Server/WebSocketBehavior.cs
index 2a053ffe..fd675e37 100644
--- a/websocket-sharp/Server/WebSocketBehavior.cs
+++ b/websocket-sharp/Server/WebSocketBehavior.cs
@@ -521,8 +521,12 @@ namespace WebSocketSharp.Server
     /// </param>
     protected void Send (string data)
     {
-      if (_websocket != null)
-        _websocket.Send (data);
+      if (_websocket == null) {
+        var msg = "The current state of the connection is not Open.";
+        throw new InvalidOperationException (msg);
+      }
+
+      _websocket.Send (data);
     }
 
     /// <summary>