We have synthesized single crystals of isotopically enriched Rb3C60. The very sharp superconducting transitions in single crystal superconducting fullerides allow us to determine the isotope effect on the superconducting transition temperature, Tc, with unprecedented accuracy. We find that the carbon isotope shift exponent αcarbon for 99% 13C substitution is 0.21±0.012, significantly smaller than other values reported in the literature1-6. This, coupled with the near-zero rubidium isotope effect reported by B. Burk et. al. 7, should place considerable constraints on any theoretical model of superconductivity in the alkali fullerides.