diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 8b007dea..de947fda 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -1412,7 +1412,22 @@ namespace WebSocketSharp.Net
     /// </exception>
     public static bool IsRestricted (string headerName, bool response)
     {
-      headerName = checkName (headerName);
+      if (headerName == null)
+        throw new ArgumentNullException ("headerName");
+
+      if (headerName.Length == 0)
+        throw new ArgumentException ("An empty string.", "headerName");
+
+      headerName = headerName.Trim ();
+
+      if (headerName.Length == 0)
+        throw new ArgumentException ("A string of spaces.", "headerName");
+
+      if (!headerName.IsToken ()) {
+        var msg = "It contains an invalid character.";
+
+        throw new ArgumentException (msg, "headerName");
+      }
 
       return isRestricted (headerName, response);
     }