diff --git a/gtk2_ardour/grid_lines.cc b/gtk2_ardour/grid_lines.cc
index d4591b9e72..978c3c51df 100644
--- a/gtk2_ardour/grid_lines.cc
+++ b/gtk2_ardour/grid_lines.cc
@@ -53,7 +53,7 @@ GridLines::hide ()
 }
 
 void
-GridLines::draw (std::vector<Ruler::Mark>     marks)
+GridLines::draw (std::vector<Ruler::Mark> const & marks)
 {
 	lines.clear();
 
diff --git a/gtk2_ardour/grid_lines.h b/gtk2_ardour/grid_lines.h
index 2be5a00b40..216e171e35 100644
--- a/gtk2_ardour/grid_lines.h
+++ b/gtk2_ardour/grid_lines.h
@@ -28,7 +28,7 @@ public:
 	GridLines (ArdourCanvas::Container* group, double screen_height);
 	~GridLines ();
 
-	void draw (std::vector<ArdourCanvas::Ruler::Mark> marks);
+	void draw (std::vector<ArdourCanvas::Ruler::Mark> const & marks);
 
 	void show();
 	void hide();