diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index 83514fce..481f06c0 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -875,7 +875,7 @@ namespace WebSocketSharp
           continue;
         }
 
-        if (separator.Contains (c)) {
+        if (separator.IndexOf (c) > -1) {
           if (quoted)
             continue;