diff --git a/src/gui.c b/src/gui.c
index 6f1d8f5..31ddd03 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -4741,7 +4741,7 @@
     int
 gui_get_lightness(guicolor_T pixel)
 {
-    long_u	rgb = gui_mch_get_rgb(pixel);
+    long_u	rgb = (long_u)gui_mch_get_rgb(pixel);
 
     return  (int)(  (((rgb >> 16) & 0xff) * 299)
 		   + (((rgb >> 8) & 0xff) * 587)