Notice: Undefined variable: sort in SearchApiElasticsearchService->buildSearchQuery()

When using Search API Views, the following errors occur when trying to add a sort.

Notice: Undefined variable: sort in SearchApiElasticsearchService->buildSearchQuery() (line 1043 of /var/www/example.com/drupal/sites/all/modules/search_api_elasticsearch/service.inc).
Recoverable fatal error: Argument 1 passed to Elastica_Query::setSort() must be an array, null given, called in /var/www/example.com/drupal/sites/all/modules/search_api_elasticsearch/service.inc on line 1043 and defined in Elastica_Query->setSort() (line 130 of /var/www/example.com/drupal/sites/all/libraries/Elastica/lib/Elastica/Query.php).