Recoverable fatal error when trying to upgrade from Drupal 7 to 8 through the Migrate Drupal UI

any suggestions?

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /drupal8/batch?id=2&op=do_nojs&op=do
StatusText: OK
ResponseText: Recoverable fatal error: Argument 1 passed to DrupaloptionsPluginFieldFieldTypeListItemBase::structureAllowedValues() must be of the type array, string given, called in /var/www/drupal/drupal8/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php on line 254 and defined in DrupaloptionsPluginFieldFieldTypeListItemBase::structureAllowedValues() (line 307 of /var/www/drupal/drupal8/core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php).
Drupal version: 


Source: https://www.drupal.org/taxonomy/term/21/feed