diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index ce9a95a3..ddd531d1 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -175,7 +175,9 @@ namespace WebSocketSharp
           var nread = stream.EndRead (ar);
           if (nread < 1) {
             // EOF/Disconnect before reading specified number of bytes.
-            completed (buffer.SubArray (0, offset));
+            if (completed != null)
+              completed (buffer.SubArray (0, offset));
+
             return;
           }