diff --git a/libmount/src/context_umount.c b/libmount/src/context_umount.c
index 36ed435cf5..17ec1f0a48 100644
--- a/libmount/src/context_umount.c
+++ b/libmount/src/context_umount.c
@@ -150,7 +150,7 @@ static int is_associated_fs(const char *devname, struct libmnt_fs *fs)
 	int flags = 0;
 
 	/* check if it begins with /dev/loop */
-	if (strncmp(devname, _PATH_DEV_LOOP, sizeof(_PATH_DEV_LOOP)))
+	if (strncmp(devname, _PATH_DEV_LOOP, sizeof(_PATH_DEV_LOOP) - 1))
 		return 0;
 
 	src = mnt_fs_get_srcpath(fs);