AI plays chimeras with nothing in play. Presumably it's thinking one or both of "creature in play with momentum is good" or "creature in play with gpull is good".
A check for whichever-is-missing of "if health = 0, gpull value = 0" and "if attack = 0, momentum value = 0" seems likely to fix it.
edit: fippe looked at code and momentum seems to be a flat boost in value, so that's probably why