WWW class is empty on iOS

I have some networking code using Unity's WWW class that reads data from a remote server. It works fine in the editor, but when I try it from an iPad the www object is completely empty - no headers, no error, nothing. Here's a stripped down version of what I'm doing:

 WWW www;
 
 public void StartLogin (string username, string password)
 {    
     WWWForm form = new WWWForm();
     form.AddField("username", username);
     
     Hashtable headers = form.headers;
     
     headers["username"] = username;
     headers["password"] = password;
     headers["deviceId"] = 2; 
     
     www = new WWW(loginUrl, form.data, headers);
     //StartCoroutine(WaitForLoginRequest());        
 }
 
 void Update()
 {
     if (www != null && www.isDone)
     {
         DoLoginRequest();
     }
 }
 
 void DoLoginRequest()
 {
         //Code goes here

         www = null;
 }

As you can see, I started using coroutines, but switched over to a simpler version using Update (didn't help). I know the connection is getting through to the server because it's logging an error 412 - I can fix that, but I'm more worried about where all my information is going on the iOS side.