diff --git a/headers/os/interface/ChannelSlider.h b/headers/os/interface/ChannelSlider.h
index 8560b362da..3f6a14e6aa 100644
--- a/headers/os/interface/ChannelSlider.h
+++ b/headers/os/interface/ChannelSlider.h
@@ -149,7 +149,7 @@ virtual	void _Reserved_BChannelSlider_7(void *, ...);
 		void DrawThumbFrame(
 				BView * where,
 				const BRect & area);
-		bool Vertical();
+		bool Vertical() const;
 		void Redraw();
 		void MouseMovedCommon(BPoint , BPoint );
 };
diff --git a/src/kits/interface/ChannelSlider.cpp b/src/kits/interface/ChannelSlider.cpp
index deb7eee56a..2ced018b68 100644
--- a/src/kits/interface/ChannelSlider.cpp
+++ b/src/kits/interface/ChannelSlider.cpp
@@ -449,7 +449,7 @@ BChannelSlider::DrawThumbFrame(BView *where, const BRect &area)
 
 
 bool
-BChannelSlider::Vertical()
+BChannelSlider::Vertical() const
 {
 	return fVertical;
 }