Hook_node_grants returns incorrect data structure

hook_node_grants() is returning the wrong data structure based on the hook documentation. The module is returning the following structure:

$grants['simple_access'] = array(
  1 => array('gid' => 1'),
);

When it should be returning:

$grants['simple_access'] = array(1);

This problem manifests itself when the simple_access module is used in conjunction with the apachesolr_access module. The combination generates an invalid Solr query due to the $grant['simple_access'] value having a nested array.