diff --git a/mtk/mtk/mtk-rectangle.c b/mtk/mtk/mtk-rectangle.c
index 7b9b0f213..70cf62c6c 100644
--- a/mtk/mtk/mtk-rectangle.c
+++ b/mtk/mtk/mtk-rectangle.c
@@ -286,7 +286,7 @@ mtk_rectangle_contains_rect (const MtkRectangle *outer_rect,
  * Returns: Return a graphene_rect_t created from `rect`
  */
 graphene_rect_t
-mtk_rectangle_to_graphene_rect (MtkRectangle *rect)
+mtk_rectangle_to_graphene_rect (const MtkRectangle *rect)
 {
   return (graphene_rect_t) {
            .origin = {
diff --git a/mtk/mtk/mtk-rectangle.h b/mtk/mtk/mtk-rectangle.h
index e8c5f59ab..0c9215fba 100644
--- a/mtk/mtk/mtk-rectangle.h
+++ b/mtk/mtk/mtk-rectangle.h
@@ -117,7 +117,7 @@ gboolean mtk_rectangle_contains_rect (const MtkRectangle *outer_rect,
                                       const MtkRectangle *inner_rect);
 
 MTK_EXPORT
-graphene_rect_t mtk_rectangle_to_graphene_rect (MtkRectangle *rect);
+graphene_rect_t mtk_rectangle_to_graphene_rect (const MtkRectangle *rect);
 
 MTK_EXPORT
 void mtk_rectangle_from_graphene_rect (const graphene_rect_t *rect,