The word ‘whomever’ has no postag. According to Merriam Webster’s dictionary, it is a pronoun (objective case of ‘whoever’).
tagset.txt has this line:
WP wh-pronoun: that, whatever, what, whatsoever, whosoever, who, whom, whoever, which (no other words)
Although I think that ‘whomever’ should have postag WP, the line tells me that no other words are permitted.
What postag should I use?