diff --git a/lib/roken/strcasecmp.c b/lib/roken/strcasecmp.c
index cfca524b7..b2425c989 100644
--- a/lib/roken/strcasecmp.c
+++ b/lib/roken/strcasecmp.c
@@ -46,13 +46,13 @@ RCSID("$Id$");
 int
 strcasecmp(const char *s1, const char *s2)
 {
-    while(toupper(*s1) == toupper(*s2)) {
+    while(toupper((unsigned char)*s1) == toupper((unsigned char)*s2)) {
 	if(*s1 == '\0')
 	    return 0;
 	s1++;
 	s2++;
     }
-    return toupper(*s1) - toupper(*s2);
+    return toupper((unsigned char)*s1) - toupper((unsigned char)*s2);
 }
 
 #endif