The growing popularity of small cells is driving cellular networks of yesterday towards heterogeneity and randomness. Soon, hundreds of unplanned user deployed femtocells and tens of operator managed picocells will coexist in a typical macrocell. One of the natural ways to model base station (BS) locations in such heterogeneous cellular networks, or HetNets, is by using random spatial models. While sufficient progress has been made in modeling single-antenna HetNets, our focus in this paper is on multi-antenna HetNets for which the modeling tools are not well developed. Assuming K classes of BSs, which may differ in terms of transmit power, target signal to interference ratio (SIR), deployment density, number of transmit antennas and multi-antenna technique, we derive an upper bound on the coverage probability using tools from stochastic geometry. We show that the bound can be reduced to closed form in certain cases of interest and is tight down to very low target SIRs.