Confusing BIND with CNAMEs

Given the zone:

subdomain IN NS nameserver
nameserver IN CNAME nameserver.otherserver

in a BIND server that is both recursive and authoritative, requests without RD (recursion desired) return the NS record, while requests with RD return SERVFAIL. Changing it to:

subdomain IN NS nameserver.otherserver

fixes the problem, though both resolution trees are perfectly valid.