osm_type query parameter is ignored

When I call http://open.mapquestapi.com/nominatim/v1/search.php, it seems that osm_type query parameter is completely ignored. Results are neither filtered by this parameter, nor they are sorted by it. I have tried many different values like N, n, Node, node but none of them seems to work. Is it a bug in the API? Or is the name of the parameter different than the one mentioned in the documention?