diff --git a/cache.h b/cache.h
index 7109765748..f9c7a60384 100644
--- a/cache.h
+++ b/cache.h
@@ -328,6 +328,7 @@ struct index_state {
 	struct ewah_bitmap *fsmonitor_dirty;
 	struct mem_pool *ce_mem_pool;
 	struct progress *progress;
+	struct repository *repo;
 };
 
 /* Name hashing */
diff --git a/repository.c b/repository.c
index a4174ddb06..c98298acd0 100644
--- a/repository.c
+++ b/repository.c
@@ -264,6 +264,12 @@ int repo_read_index(struct repository *repo)
 	if (!repo->index)
 		repo->index = xcalloc(1, sizeof(*repo->index));
 
+	/* Complete the double-reference */
+	if (!repo->index->repo)
+		repo->index->repo = repo;
+	else if (repo->index->repo != repo)
+		BUG("repo's index should point back at itself");
+
 	return read_index_from(repo->index, repo->index_file, repo->gitdir);
 }