diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index fd320122..e0d5779b 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -1525,7 +1525,8 @@ namespace WebSocketSharp
       if (idx >= 10)
         return false;
 
-      return value.Substring (0, idx).IsPredefinedScheme ();
+      var schm = value.Substring (0, idx);
+      return schm.IsPredefinedScheme ();
     }
 
     /// <summary>