Simplified models of dark matter have been widely adopted to give a unified framework that describes dark matter interactions in direct detection, indirect detection and LHC experiments. While this framework gives good complementarity between different experiments, simplified models are not complete theories and suffer from various theoretical issues. One issue is the presence of gauge anomalies in the Z' mediated theories, potentially destroying gauge invariance. We explore the conditions that anomaly cancellation imposes on simplified models, finding that you have to either couple to the SM leptons, or introduce several exotic fermions including an SU(2) doublet. In the case of lepton couplings, the constraints coming from dilepton searches will be much stronger than those coming from the traditional dijet and monojet searches. Separately, Z' models have been proposed with flavour changing interactions in order to explain the recent excesses observed in semileptonic B meson decays. Building a gauge invariant, anomaly-free theory requires extra couplings and/or particles, including a possible dark matter candidate.