diff --git a/tests/server/tftpd.c b/tests/server/tftpd.c
index 7dafd78a8db6aa26d1395f06d1a0dd24ad89d416..d4a4b53c5d6dfb115810179c7242d395c23a140c 100644
--- a/tests/server/tftpd.c
+++ b/tests/server/tftpd.c
@@ -620,7 +620,7 @@ again:
   fprintf(server, "filename: %s\n", filename);
 
   for (cp = mode; *cp; cp++)
-    if (isupper((int)*cp))
+    if(ISUPPER(*cp))
       *cp = (char)tolower((int)*cp);
 
   /* store input protocol */