Bright basal reflections from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) have been proposed to be consistent with permittivities characteristic of a wet material beneath the south polar layered deposits (SPLD). The characterization of a recently formed impact crater highlight the existence of a several meters thick ice-poor layer associated to a unit blanketing a large portion of the SPLD. We revise the radar propagation model used to invert the basal permittivity by including a surficial thin layer. We find that the inverted basal permittivity is highly sensitive to the properties of such a layer, with solutions ranging from common dry rocks to an unambiguously wet base. We advocate toward a better characterization of the surficial cover to assess the wet or dry nature for the base, and possibly reconcile most of the literature on the topic.