diff --git a/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.cpp b/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.cpp
index 61b6cf21116828c8145ed7e6a06f956432e74953..57d1c97ceb5b416235c41d89d017154f817dda0d 100644
--- a/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.cpp
+++ b/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.cpp
@@ -188,15 +188,15 @@ bool VisiblyPushdownNPDA::removeTransition(const State& from, const alphabet::Sy
 	return localTransitions[key].erase(to);
 }
 
-const std::map<std::pair<State, alphabet::Symbol>, std::set<std::pair<State, alphabet::Symbol> > > VisiblyPushdownNPDA::getCallTransitions() const {
+const std::map<std::pair<State, alphabet::Symbol>, std::set<std::pair<State, alphabet::Symbol> > >& VisiblyPushdownNPDA::getCallTransitions() const {
 		return callTransitions;
 }
 
-const std::map<std::tuple<State, alphabet::Symbol, alphabet::Symbol>, std::set<State> > VisiblyPushdownNPDA::getReturnTransitions() const {
+const std::map<std::tuple<State, alphabet::Symbol, alphabet::Symbol>, std::set<State> >& VisiblyPushdownNPDA::getReturnTransitions() const {
 		return returnTransitions;
 }
 
-const std::map<std::pair<State, alphabet::Symbol>, std::set<State> > VisiblyPushdownNPDA::getLocalTransitions() const {
+const std::map<std::pair<State, alphabet::Symbol>, std::set<State> >& VisiblyPushdownNPDA::getLocalTransitions() const {
 		return localTransitions;
 }
 
diff --git a/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.h b/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.h
index e446916aafaf65999e2ef9f6a9862a5c85e84d50..132293068f5daafd26116ddf81fba10c49a6c7e5 100644
--- a/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.h
+++ b/alib2data/src/automaton/PDA/VisiblyPushdownNPDA.h
@@ -98,11 +98,11 @@ public:
 	 */
 	bool removeTransition(const State& current, const alphabet::Symbol& input, const State& next);
 
-	const std::map<std::pair<State, alphabet::Symbol>, std::set<std::pair<State, alphabet::Symbol> > > getCallTransitions() const;
+	const std::map<std::pair<State, alphabet::Symbol>, std::set<std::pair<State, alphabet::Symbol> > >& getCallTransitions() const;
 
-	const std::map<std::tuple<State, alphabet::Symbol, alphabet::Symbol>, std::set<State> > getReturnTransitions() const;
+	const std::map<std::tuple<State, alphabet::Symbol, alphabet::Symbol>, std::set<State> >& getReturnTransitions() const;
 
-	const std::map<std::pair<State, alphabet::Symbol>, std::set<State> > getLocalTransitions() const;
+	const std::map<std::pair<State, alphabet::Symbol>, std::set<State> >& getLocalTransitions() const;
 
 	virtual bool operator<(const alib::ObjectBase& other) const;
 	virtual bool operator==(const alib::ObjectBase& other) const;