diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 8062a3fb..1fb60dc7 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -855,7 +855,7 @@ namespace WebSocketSharp.Net
       }
 
       if (!value.IsText ()) {
-        var msg = "It contains an invalid character.";
+        var msg = "The value contains an invalid character.";
 
         throw new ArgumentException (msg, "value");
       }