[en] We study the electronic structure of the SmFeAsO_1−xF_x alloy by means of first-principle calculations. We find that, contrary to common believe, F-doping does not change the charge balance between electrons and holes free-carriers in SmFeAsO_1−xF_x. For energies within a narrow energy range accross , the effect of F-doping on the band structure dispersion is tiny in both the paramagnetic and stripe antiferromagnetic phase. The charge balance between the conducting FeAs-layer and the SmO_1−xF_x charge reservoir layer is not influenced by the compositional change. The additional charge carried by fluorine, with respect to the oxygen, is compensated by a change in the oxidation state of the Sm ion from 3+  to 2+. A comparison with the SmFe_1−xCo_xAsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site. (paper)