res_findzonecut2() problem/question

[Ronald F. Guilmette]

I'm calling res_findzonecut2() in the following way:

   r = resfindzonecut2 (res, "120.40.64.in-addr.arpa", ns_c_in, RES_IPV4ONLY,
                        zone_name, zone_size, NULL, 0);

I just want to get the name of the zone that "120.40.64.in-addr.arpa" is
part of.

Unfortunately, the above call returns -1 and errno is set to EPROTOTYPE
(Protocol wrong type for socket).  I have no understanding of this error
or how to work around it.  Can anyone explain to me why the error is
occuring and what I need to do in order to find the containing zone for
"120.40.64.in-addr.arpa" ?