diff --git a/fs/adfs/dir_f.c b/fs/adfs/dir_f.c
index bbfc862..b9b2b27 100644
--- a/fs/adfs/dir_f.c
+++ b/fs/adfs/dir_f.c
@@ -53,7 +53,7 @@
 {
 	char *old_buf = buf;
 
-	while (*ptr >= ' ' && maxlen--) {
+	while ((unsigned char)*ptr >= ' ' && maxlen--) {
 		if (*ptr == '/')
 			*buf++ = '.';
 		else