diff --git a/strbuf.c b/strbuf.c
index bc3a0802ea..65b4cf4343 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -399,6 +399,8 @@ int strbuf_branchname(struct strbuf *sb, const char *name)
 int strbuf_check_branch_ref(struct strbuf *sb, const char *name)
 {
 	strbuf_branchname(sb, name);
+	if (name[0] == '-')
+		return CHECK_REF_FORMAT_ERROR;
 	strbuf_splice(sb, 0, 0, "refs/heads/", 11);
 	return check_ref_format(sb->buf);
 }