diff --git a/libs/ardour/ardour/stripable.h b/libs/ardour/ardour/stripable.h
index eb00d637b0..48cb21d6d5 100644
--- a/libs/ardour/ardour/stripable.h
+++ b/libs/ardour/ardour/stripable.h
@@ -95,7 +95,7 @@ class LIBARDOUR_API Stripable : public SessionObject,
 
 	struct LIBARDOUR_API Sorter
 	{
-		bool _mixer_order; // master is last
+		const bool _mixer_order; // master is last
 		Sorter (bool mixer_order = false) : _mixer_order (mixer_order) {}
 		bool operator() (std::shared_ptr<ARDOUR::Stripable> a, std::shared_ptr<ARDOUR::Stripable> b) const;
 	};