The free energy of sodium ion hydration calculated from computer simulations employing both the molecular potential truncation and Ewald summation methods for evaluation of long-range electrostatic interactions disagree by 20%. The discrepancy between the free energies determined by both techniques is found to be methodological, resulting from an imbalance in the molecular potential truncation scheme that biases the interaction of the water-oxygen and hydrogens with the ion unequally. A simple physical model is proposed and an analytical expression derived to correct this discrepancy.