Bass wah pedals are meant to work with the lower tones, but a regular wah pedal works just fine. Dennis Dunaway used a wah on Alice Cooper's Unfinished Sweet, and at the time there was no one tailoring wah pedals for bass. It worked out fine. Geezer Butler used a wah on N.I.B. It was likely unmodified, too.
Use your ears. If the pedal sounds good use it.