Return statement in IEnumerator Unity C#

Hi,

 I have problem in IEunmerator function Unity C#.

In the following script I want to return the function if the the string is null. But, using IEnumerator will not allow me to put return statement inside of it.

 IEnumerator DownloadData ()
 {
         WWW download = new WWW (URL, form);
         yield return download;
         
         XMLValue.PHPValues= this.gameObject;
         if (!string.IsNullOrEmpty(download.error))
         {
             ErrorPopup("Please check your internet connection.");
             return;
         }
  }

If i use

 yield return 0;
 (or)
 yield return null;
 

will execute the lines after this line. But, How could i return control from the function??