Bug applies to libasyncns release 0.8 and current git revision 68cd5a.
Both asyncns_setuserdata(...) and asyncns_getuserdata(...) contain the following line:
assert(q->asyncns = asyncns);
I believe this should not be assignment, but a check for equality, as is correctly done already in asyncns_isdone(...):
assert(q->asyncns == asyncns);
Thank you for a very useful and fine piece of free software in libasyncns!