I suppose the appropriate way to state the solution is that the mass approaches infinity as the ball is accelerated toward the speed of light. The amount of force/energy required to accelerate the ball increases as the mass increases, and therefore you can never reach the speed of light because it would require an infinite amount of energy to do so. Strictly speaking the function is not defined at a velocity of v = c, so there's no analytical calculation to make. As Borek pointed out, 1/0 isn't imaginary - it's undefined.