188-194 line of 'civicrm/CRM/Bridge/OG/Drupal.php'. $contactId will be null because the 'CRM_BRIDGE_OG_UTILS::contactID()' function call returns null.

Drupal

    static function og( &$params, $op ) {
        require_once 'CRM/Bridge/OG/Utils.php';

        $contactID = CRM_Bridge_OG_Utils::contactID( $params['uf_id'] );
        if ( ! $contactID ) {
            CRM_Core_Error::fatal( );
        }