diff --git a/init/do_mounts_dm.c b/init/do_mounts_dm.c
index 0fe9c5f7d5e9..a557c5ee00a7 100644
--- a/init/do_mounts_dm.c
+++ b/init/do_mounts_dm.c
@@ -176,7 +176,8 @@ static void __init dm_substitute_devices(char *str, size_t str_len)
 			continue;
 
 		/* Temporarily terminate with a nul */
-		candidate_end--;
+		if (*candidate_end)
+			candidate_end--;
 		old_char = *candidate_end;
 		*candidate_end = '\0';