We played this that this ability was limited to base game creature deck cards. So it means that if you would find hero, or Kemet reinforcement (C3K) you would get simply nothing just simply discard this card.
This seems harsh as it dilutes the effectiveness of the monument simply based on whether you included other expansions or not. At minimum, you should re-draw until you get a creature.
I imagine though it'd work for Heroes as well and you acquire them for free, but must still of course pay upkeep.