diff --git a/pyeris/core/Market.cpp b/pyeris/core/Market.cpp
index 032a095e8cf1908b8708c69fd904c073a598c8fa..8cc09f5fe82883193b133c066d12a4a6395e4e94 100644
--- a/pyeris/core/Market.cpp
+++ b/pyeris/core/Market.cpp
@@ -58,7 +58,15 @@ void bind_market(py::module &m) {
                 "Creates a Reservation object for the given agent, quantity, and maximum expenditure.  This method is for internal use by Market subclasses to actually create the object; external callers should call reserve() instead.  This method removes the payment from the agent ")
         .def("add_firm", &Market::addFirm, "Adds the given firm to the firms supplying output units in this market.")
         .def("remove_firm", &Market::removeFirm, "Removes the given firm from the firms supplying output units in this market.  This only needs to be called explicitly when the firm remains in the simulation; firms removed from the simulation will have this called automatically.")
-        .def("firms", &Market::firms, "Returns the set of firms that supply this market")
+        // NB: firms in the Python interface is a python set of firms, unlike the C++ interface
+        // where firms() is a (C++) unordered_set of firm *ids*
+        .def("firms", [](Market &m) {
+                py::set firms;
+                auto sim = m.simulation();
+                for (auto fid : m.firms())
+                    firms.add(py::cast(sim->agent<Firm>(fid)));
+                return firms; }, "Returns a set of firms that supply this market.")
+
         .def_readonly("output_unit", &Market::output_unit, "The bundle in terms of which output quantities are measured.")
         .def_readonly("price_unit", &Market::price_unit, "The bundle in terms of which output prices are measured.")
         ;