How loud do you actually need to play? A 15-watt guitar amp can technically handle bass without immediately blowing up, but the previous answer's concern about speaker stress is overstated - the real issue is that your amp's speaker just isn't designed to reproduce low frequencies cleanly, so you'll get a muddy, flabby tone at best and potential speaker damage if you crank it. You could use it as a practice amp at bedroom volumes without major risk, but if you're planning to gig or rehearse with others, you'd notice pretty quick that it's not cutting it. Honestly, a used bass amp isn't expensive these days, and it'll save your Fender in the long run.