diff --git a/websocket-sharp/Server/WebSocketBehavior.cs b/websocket-sharp/Server/WebSocketBehavior.cs
index 5637c2ee..2edb2bd2 100644
--- a/websocket-sharp/Server/WebSocketBehavior.cs
+++ b/websocket-sharp/Server/WebSocketBehavior.cs
@@ -556,8 +556,12 @@ namespace WebSocketSharp.Server
     /// </param>
     protected void SendAsync (byte[] data, Action<bool> completed)
     {
-      if (_websocket != null)
-        _websocket.SendAsync (data, completed);
+      if (_websocket == null) {
+        var msg = "The current state of the connection is not Open.";
+        throw new InvalidOperationException (msg);
+      }
+
+      _websocket.SendAsync (data, completed);
     }
 
     /// <summary>