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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 A_L 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, A_L) is determined for every possible TKE and A_L. For every fragment mass ratio, differences in behavior between [nu with overbar](TKE, A_L) versus TKE and [nu with overbar](TKE) with no discrimination made with regard to A_L 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.