diff --git a/libs/gtkmm2ext/pixfader.cc b/libs/gtkmm2ext/pixfader.cc
index 6c4a511ae1..43657a5902 100644
--- a/libs/gtkmm2ext/pixfader.cc
+++ b/libs/gtkmm2ext/pixfader.cc
@@ -291,8 +291,10 @@ PixFader::on_size_allocate (Gtk::Allocation& alloc)
 
 	if (_orien == VERT) {
 		girth = alloc.get_width ();
+		span = alloc.get_height ();
 	} else {
 		girth = alloc.get_height ();
+		span = alloc.get_width ();
 	}
 
 	update_unity_position ();