diff --git a/builtin/mv.c b/builtin/mv.c
index 4b67bd096a..35bf1e9c71 100644
--- a/builtin/mv.c
+++ b/builtin/mv.c
@@ -71,7 +71,7 @@ static const char **internal_prefix_pathspec(const char *prefix,
 static const char *add_slash(const char *path)
 {
 	size_t len = strlen(path);
-	if (path[len - 1] != '/') {
+	if (len && path[len - 1] != '/') {
 		char *with_slash = xmalloc(st_add(len, 2));
 		memcpy(with_slash, path, len);
 		with_slash[len++] = '/';