diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 544e9f74c..3d7cc730e 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -2532,6 +2532,7 @@ 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();