diff --git a/sc2/src/port.c b/sc2/src/port.c
index f07f432a9..0f4394535 100644
--- a/sc2/src/port.c
+++ b/sc2/src/port.c
@@ -39,7 +39,7 @@ strupr (char *str)
 	ptr = str;
 	while (*ptr)
 	{
-		*ptr = toupper (*ptr);
+		*ptr = (char) toupper (*ptr);
 		ptr++;
 	}
 	return str;