customPost profile passes array instead of id to filterUFGroups

CRM/Core/BAO/UFGroup.php filterUFGroups is getting passed an array instead of an ID for parameter 1 but only on the customPost profile (2nd profile option). Tracked this down to CRM/Event/Form/Register.php function buildQuickForm where

$this->buildCustom( $this->_values['custom_post_id'] , 'customPost' );

contains an array instead of an id. For my own fix I replaced that line with the following two lines:

$custom_post_id = CRM_Utils_Array::value( 0, $this->_values['custom_post_id'], '');
$this->buildCustom( $custom_post_id , 'customPost' );

but I anticipate there is some other way this should be solved.