diff --git a/Fsm.cpp b/Fsm.cpp
index 9b932ca..4496cc6 100644
--- a/Fsm.cpp
+++ b/Fsm.cpp
@@ -26,8 +26,9 @@ State::State(void (*on_enter)(), void (*on_state)(), void (*on_exit)())
 
 Fsm::Fsm(State* initial_state)
 : m_current_state(initial_state),
-  m_transitions(NULL),
+  m_transitions(nullptr),
   m_num_transitions(0),
+  m_timed_transitions(nullptr),
   m_num_timed_transitions(0),
   m_initialized(false)
 {
diff --git a/Fsm.h b/Fsm.h
index afbb3dd..aa3c455 100644
--- a/Fsm.h
+++ b/Fsm.h
@@ -26,7 +26,7 @@
 
 struct State
 {
-  State(void (*on_enter)(), void (*on_state)(), void (*on_exit)());
+  State(void (*on_enter)(), void (*on_state)() = nullptr, void (*on_exit)() = nullptr);
   void (*on_enter)();
   void (*on_state)();
   void (*on_exit)();
@@ -40,10 +40,10 @@ class Fsm
   ~Fsm();
 
   void add_transition(State* state_from, State* state_to, int event,
-                      void (*on_transition)());
+                      void (*on_transition)() = nullptr);
 
   void add_timed_transition(State* state_from, State* state_to,
-                            unsigned long interval, void (*on_transition)());
+                            unsigned long interval, void (*on_transition)() = nullptr);
 
   void check_timed_transitions();