diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index bbcf58417..014dafcd2 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -2526,6 +2526,7 @@ struct RTLIL::CaseRule : public RTLIL::AttrObject
 	std::vector<RTLIL::SigSpec> compare;
 	std::vector<RTLIL::SyncAction> actions;
 	std::vector<RTLIL::SwitchRule*> switches;
+	TwineRef compare_src = Twine::Null;
 
 	~CaseRule();