We use highly oriented pyrolytic graphite (HOPG) with a submonolayer coverage of C₆₀ and monitor the step height from C₆₀ to HOPG as a function of dc bias for non-contact atomic force microscopy (NC-AFM). We find a strong dependence of the step height on dc bias between tip and sample surface. The step height is modified by uncompensated electrostatic forces, resulting from the work function difference of  meV between C₆₀ and HOPG, as determined by Kelvin probe force microscopy (KPFM) measurements. In comparison, steps from C₆₀ to C₆₀ or from HOPG to HOPG show no dependence on dc bias. This effect is well described by simulations of the electrostatic interaction between tip and sample. The results clearly demonstrate the influence of uncompensated electrostatic forces on height measurements in NC-AFM, and show that correct height determination requires the use of KPFM with active control of the bias voltage.