Explanation of a resolver.c error message

[Keith Christian]

Could anyone offer an explanation for what condition(s) trigger this
error in older, out of date versions of BIND, specifically, BIND
9.5.1b1 ?

resolver.c:5617: REQUIRE((((query) != ((void *)0)) && (((const
isc__magic_t *)(query))->magic == ((('Q') << 24 | ('!') << 16 | ('!')
<< 8 | ('!')))))) failed

Is this related to a type of query, or some other event?

Thanks!

==========Keith