Correct behavior with If-Match the header?

According to RFC 2616, generation of entity tags by HTTP servers is optional. However, I couldn't find what a conditionally compliant HTTP server should do if it receives an If-Match (or If-None-Match) header. Should it just ignore those headers or should it respond with 412 Precondition Failed?

UPD: Just to clarify, I'm assuming that the server in question does not support entity tags.