Recently, a spatially extended excess of gamma rays collected by the Fermi-LAT from the inner region of the Milky Way has been discovered by different groups and with increasingly sophisticated techniques of data analysis. I will show that spectral properties and morphology of the excess are robust against large variations of the Galactic diffuse emission, which represents the largest background for such an analysis. I will quantify the systematic uncertainties affecting the excess and demonstrate that the proper treatment of background modelling uncertainties, which dominate over instrumental systematics, allows more freedom for models fitting the excess.
