A control law is derived for the Zero-Inertia (ZI) powertrain, a non-linear system with an ill-defined relative degree. Classical non-linear feedback control of this system is impossible. The control law designed in this article uses approximate input-output linearisation. Stability of the closed loop system is proved using Lyapunov's direct method and the theorem of Hartman-Grobman. Simulations show that the new control law results in good tracking behaviour.