Cannot modify finalUrl of ExpandedTextAd

[Emily Jia]

I am trying to use the ad group ad service to modify the finalUrl of an expanded text ad. I call mutate with the ad group ad service , but I get 

'faultMessage': '[AdError.CANNOT_MODIFY_AD @ operations[0].operand.ad.finalUrls]'

Here are the operations:

  operations = [{

      'operator': 'SET',

      'operand': {

        'xsi_type': 'AdGroupAd',

          'adGroupId': ##,

          'ad': {

              'id': ##,

            'finalUrls': [##]

          },

        'status': 'ENABLED'

      }

  }]