Translation finished notification error

Stack trace
E_ERROR: Class 'HttpResponse' not found
in tmgmt_oht_callback called at sites/all/modules/contrib/tmgmt_oht/tmgmt_oht.module (96)
in tmgmt_oht_callback called at ? (?) in call_user_func_array called at includes/menu.inc (527)
in menu_execute_active_handler called at index.php (21)

This appears to be happening when receiving notification of a completed translation.