The non-hydrostatic contribution to the oblateness is calculated based on the single assumption that non-hydrostatic contributions to the moments-of-inertia are axially symmetric about the principal axis in the direction of the Tharsis dome. The correction to J2 is thereby -2J22, in numerical value equal to that obtained by Reasenberg by a more complicated technique. The resulting C/MR2 is 0.365. The error in these values depends entirely on the extent to which mass irregularities depart from axial symmetry around Tharsis, which plausibly would not be great.