Setting txtUrl in Image Properties dialog throws an error

This is for CKEditor 3.0.1 and 3.0.2

We are using our own image upload component and are just entering the image path into the 'Image Properties > Image Info > Url' text box.

To save time, I wanted to pre-populate the text box with the first part of the url, so we would only have to type in the filename.

I have been trying to use a version of the dialog api example given in the download:

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';
      }
});