We discuss long-distance QED contributions to B^0 → D^+τν and B^− →D^0τν relative to B^0 → D^+µν and B^− → D^0µν, respectively, in the Standard Model. We point out that the QED corrections to the ratios R(D^+) and R(D^0) are not the same and not negligible. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles; muon mode is a relativistic decay but tau mode is not.