diff --git a/include/Lexesis/automata.h b/include/Lexesis/automata.h
index 7699e15..a6335b1 100644
--- a/include/Lexesis/automata.h
+++ b/include/Lexesis/automata.h
@@ -26,12 +26,12 @@ namespace lxs {
 
     struct NFA : public Automaton {
         std::map<State, std::map<char, std::set<State> > > delta;
-        virtual std::set<State> eClose(State);
+        virtual std::set<State> eClose(State) const;
     };
 
     struct ENFA : public NFA {
         std::map<State, std::set<State> > epsilonTransitions;
-        virtual std::set<State> eClose(State);
+        virtual std::set<State> eClose(State) const;
     };
 
     std::string toDot(const DFA& d);