diff --git a/libs/ardour/luabindings.cc b/libs/ardour/luabindings.cc
index 831700fb34..fb75fec380 100644
--- a/libs/ardour/luabindings.cc
+++ b/libs/ardour/luabindings.cc
@@ -234,7 +234,7 @@ CLASSINFO(UIConfiguration);
 
 
 /* this needs to match gtk2_ardour/luasignal.h */
-CLASSKEYS(std::bitset<49ul>); // LuaSignal::LAST_SIGNAL
+CLASSKEYS(std::bitset<50ul>); // LuaSignal::LAST_SIGNAL
 
 CLASSKEYS(void);
 CLASSKEYS(float);