diff --git a/object.c b/object.c
index b2319f6246..69fbbbf504 100644
--- a/object.c
+++ b/object.c
@@ -163,6 +163,8 @@ void *object_as_type(struct object *obj, enum object_type type, int quiet)
 	if (obj->type == type)
 		return obj;
 	else if (obj->type == OBJ_NONE) {
+		if (type == OBJ_COMMIT)
+			((struct commit *)obj)->index = alloc_commit_index();
 		obj->type = type;
 		return obj;
 	}