  Recently, a spatially extended excess of gamma rays collected by the
Fermi-LAT from the inner region of the Milky Way has been detected by different
groups and with increasingly sophisticated techniques. Yet, any final
conclusion about the morphology and spectral properties of such an extended
diffuse emission are subject to a number of potentially critical uncertainties,
related to the high density of cosmic rays, gas, magnetic fields and abundance
of point sources. We will present a thorough study of the systematic
uncertainties related to the modelling of diffuse background and to the
propagation of cosmic rays in the inner part of our Galaxy. We will test a
large set of models for the Galactic diffuse emission, generated by varying the
propagation parameters within extreme conditions. By using those models in the
analysis of Fermi-LAT data as Galactic foreground, we will show that the
gamma-ray excess survives and we will quantify the uncertainties affecting the
excess morphology and energy spectrum.
