The average total prompt neutron multiplicity [nu with overbar] of 252Cf spontaneous fission is investigated as a function of the total kinetic energy TKE and the mass split of the fragments through the code FIFRELIN. This Monte Carlo device, already described in a previous work, aims at simulating the neutron evaporation from fission fragments. The observables and TKE and the light fragment mass AL are recorded from a sample of 107 fission events. The analyzed results show a value for the inverse of the slope [[partial differential][nu with overbar](TKE)/[partial differential]TKE]-1 equal to -11.0 MeV/n. In addition to this, the average number of neutrons per fission [nu with overbar](TKE, AL) is determined for every possible TKE and AL. For every fragment mass ratio, differences in behavior between [nu with overbar](TKE, AL) versus TKE and [nu with overbar](TKE) with no discrimination made with regard to AL are observed. Those differences are explained by the TKE dependency of fission yield. The approximation consisting of ignoring this TKE dependency of mass yield when calculating the [nu with overbar](TKE) slope is discussed. We estimate that such a calculation could lead to a significant bias on the absolute value of [partial differential][nu with overbar](TKE)/[partial differential]TKE and could explain the discrepancies between calculations found in the literature.