diff --git a/cpp/FactorGraph-inl.h b/cpp/FactorGraph-inl.h
index bfedfa33c..f23696cde 100644
--- a/cpp/FactorGraph-inl.h
+++ b/cpp/FactorGraph-inl.h
@@ -272,7 +272,7 @@ void FactorGraph<Factor>::associateFactor(int index, sharedFactor factor) {
 
 /* ************************************************************************* */
 template<class Factor> template <class Key>
-map<Key,Key> FactorGraph<Factor>::findMinimumSpanningTree() const {
+PredecessorMap<Key> FactorGraph<Factor>::findMinimumSpanningTree() const {
 
 	SDGraph<Key> g = gtsam::toBoostGraph<FactorGraph<Factor>, sharedFactor, Key>(*this);
 
@@ -281,7 +281,7 @@ map<Key,Key> FactorGraph<Factor>::findMinimumSpanningTree() const {
 	prim_minimum_spanning_tree(g, &p_map[0]);
 
 	// convert edge to string pairs
-	map<Key, Key> tree;
+	PredecessorMap<Key> tree;
 	typename SDGraph<Key>::vertex_iterator itVertex = boost::vertices(g).first;
 	typename vector<typename SDGraph<Key>::Vertex>::iterator vi;
 	for (vi = p_map.begin(); vi!=p_map.end(); itVertex++, vi++) {
diff --git a/cpp/FactorGraph.h b/cpp/FactorGraph.h
index c49a669cd..af54edcbf 100644
--- a/cpp/FactorGraph.h
+++ b/cpp/FactorGraph.h
@@ -120,7 +120,7 @@ namespace gtsam {
 		/**
 		 * find the minimum spanning tree.
 		 */
-		template<class Key> std::map<Key, Key> findMinimumSpanningTree() const;
+		template<class Key> PredecessorMap<Key> findMinimumSpanningTree() const;
 
 		/**
 		 * Split the graph into two parts: one corresponds to the given spanning tre,