diff --git a/src/pymatching/sparse_blossom/driver/user_graph.pybind.cc b/src/pymatching/sparse_blossom/driver/user_graph.pybind.cc
index daf1b7e0..0708b5d8 100644
--- a/src/pymatching/sparse_blossom/driver/user_graph.pybind.cc
+++ b/src/pymatching/sparse_blossom/driver/user_graph.pybind.cc
@@ -227,6 +227,10 @@ void pm_pybind::pybind_user_graph_methods(py::module &m, py::class_<pm::UserGrap
             }
             std::cout << std::endl;
             std::cout << "mwpm.flooder.negative_weight_sum: " << mwpm.flooder.negative_weight_sum << std::endl;
+            if (mwpm.flooder.negative_weight_sum != 0) {
+                throw std::invalid_argument(
+            "Decoding to matched detection events not supported for graphs containing edges with negative weights.");
+            }
             pm::decode_detection_events_to_match_edges(mwpm, detection_events_vec);
             std::cout << "B3" << std::endl;
             py::ssize_t num_edges = (py::ssize_t)(mwpm.flooder.match_edges.size());