diff --git a/output.c b/output.c
index 42ed10e..6ec3c11 100644
--- a/output.c
+++ b/output.c
@@ -207,7 +207,7 @@ static void
 count_surface_iterator(struct cg_output *output, struct wlr_surface *surface,
                        struct wlr_box *_box, void *data) {
 	size_t *n = data;
-	n++;
+	(*n)++;
 }
 
 static bool