diff --git a/builtin/worktree.c b/builtin/worktree.c
index e866844685..cce555cbbc 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -262,7 +262,7 @@ static int add_worktree(const char *path, const char *refname,
 	 */
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/HEAD", sb_repo.buf);
-	write_file(sb.buf, sha1_to_hex(null_sha1));
+	write_file(sb.buf, "%s", sha1_to_hex(null_sha1));
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/commondir", sb_repo.buf);
 	write_file(sb.buf, "../..");