diff --git a/lib/roken/strlwr.c b/lib/roken/strlwr.c
index 5e3e561c8..f60a17b9b 100644
--- a/lib/roken/strlwr.c
+++ b/lib/roken/strlwr.c
@@ -47,7 +47,7 @@ strlwr(char *str)
   char *s;
 
   for(s = str; *s; s++)
-    *s = tolower(*s);
+    *s = tolower((unsigned char)*s);
   return str;
 }
 #endif