Assertion error when SOA is missing in unsigned zone

First I prepared all files unsigned and signed, then I started to break stuff and test. I'm using BIND and OpenDNSSEC for signing and both failed to sign the zone when the SOA was missing.

OpenDNSSEC failed with the following error:
Feb 25 23:24:09 catwoman ods-signerd: signer/zone.c:493: zone_update_serial: assertion rrset failed