The perovskite stannates BaSnO3 and SrSnO3 are being actively explored for applications as transparent conductors, in power or high-frequency electronics, and as channel materials in epitaxial integration with functional perovskites. Realizing these applications requires controlled n-type doping, i.e., avoiding the formation of compensating acceptor-type defects. Here, we use density-functional theory to examine the formation of cation antisite defects. Our results indicate that antisites are not a problem in BaSnO3; however, in SrSnO3, SrSn antisites may act as compensating centers.