diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 91235216..54119988 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -793,7 +793,7 @@ namespace WebSocketSharp.Net
         return;
 
       if (headerType != _state) {
-        var msg = "This collection does not allow the header.";
+        var msg = "This instance does not allow the header.";
 
         throw new InvalidOperationException (msg);
       }