We investigate the possibility that the flattening of the fraction-mass relation (Fig. 8) is caused by the uncertainty on the stellar mass estimates. To do this, we generate X-ray AGN samples characterised by a relation f_X = C₁₁M^αwhere C₁₁ and αare free variables. After scattering the stellar mass estimates, we derive χ² corresponding to each C₁₁and α. This test shows the scatter introduced by the stellar mass uncertainty cannot explain the flattening of the f_X − M relation.