diff --git a/refs.c b/refs.c
index 9b56fa9b81..4a44e44b6a 100644
--- a/refs.c
+++ b/refs.c
@@ -1654,6 +1654,9 @@ struct ref_store *get_main_ref_store(struct repository *r)
 	if (r->refs)
 		return r->refs;
 
+	if (!r->gitdir)
+		BUG("attempting to get main_ref_store outside of repository");
+
 	r->refs = ref_store_init(r->gitdir, REF_STORE_ALL_CAPS);
 	return r->refs;
 }