dkim-filter does not properly handle verification of messages signed with a key that has been revoked in DNS (the p= flag set to an empty string).
revoked._domainkey.loopted.com. TXT "v=DKIM1; g=*; k=rsa; p="
dkim-filter -t shows the actual error:
dkim-filter: dkim.c:3725: dkim_error: Assertion `format != ((void *)0)' failed.
A backtrace is attached.