If you mistake a Polearm looking axe for a polearm, the tool tip for Serpent strike, will still calculate the damage you would do with the incorrect weapon equipped.
the damage should read as 0 or as like a X because that weapon will not work with that skill, so it won’t do any damage.
i accidentally vendored and needed to buy back my polearm, because a Bardiche axe i found, looked like a polearm, had better damage and didn’t realize it till i left town to fight