Setting txtUrl in Image Properties dialog throws an error

CKEDITOR.on( 'dialogDefinition', function( ev )
{
      // Take the dialog name and its definition from the event
      var dialogName = ev.data.name;
      var dialogDefinition = ev.data.definition;
      // Check if the definition is from the dialog we're
      // interested on (the "Image" dialog).
      if ( dialogName == 'image' )
      {
         // Get a reference to the "Image Info" tab.
         var infoTab = dialogDefinition.getContents( 'info' );
         // Set the default value for the URL field.
         var urlField = infoTab.get( 'txtUrl' );
         urlField['default'] = 'http://www.example.com';
      }
});