Necessary Data Rewind Wasn't Possible WWWForm Error

I am trying to create a login script, and when I run the code I get this error: Necessary Data Rewind Wasn't Possible. Why is this? Any help would be much appreciated.

 #pragma strict
 var formUser = ""; 
 var formPass = "";
 var formText = "";
 
 var formURL  = "http://localhost/unity/checkuser.php";
 var hash = "45gfv";
 
 function OnGUI () {
     GUILayout.BeginArea ( new Rect(Screen.width / 2 - 100, Screen.height / 2 - 50, 300, 500) );
     
     formUser = GUI.TextField (Rect (0, 0, 300, 50), formUser);
     formPass = GUI.TextField (Rect (0, 55, 300, 50), formPass);
 
     if(GUI.Button(Rect(0,110,148,50),"Login"))
     {
         Login();
     }
     if(GUI.Button(Rect(152,110,148,50),"Register"))
     {
           //register form
     }
     
     GUI.Label (Rect (0, 165, 300, 50), formText.ToString());
     
     GUILayout.EndArea();
 }
 
 
 function Login()
 {
     var form = new WWWForm();
     form.AddField("login_hash", hash);
     form.AddField("login_user", formUser);
     form.AddField("login_pass", formPass);
     
     var w = WWW(formURL, form);
     yield w;
     if(w.error != null) {
         print(w.error);
         formText = "Unexpected Error, Please Try Again.";
     }
     else {
         print("Form Ok");
         formText = w.data;
         w.Dispose();
     }
     formUser = "";
     formPass = "";
 }