diff --git a/source/core/indexes.txt b/source/core/indexes.txt
index 1eced359615..f4d7e964a8a 100644
--- a/source/core/indexes.txt
+++ b/source/core/indexes.txt
@@ -104,7 +104,7 @@ You cannot delete the index on ``_id``.
 
 The ``_id`` field is the :term:`primary key` for the collection, and
 every document *must* have a unique ``_id`` field. You may store any
-unique value in the ``_id`` field. The default value of ``_id`` is
+unique value in the ``_id`` field. The default value of ``_id`` is an
 :term:`ObjectID` on every insert() <db.collection.insert()` operation. An :term:`ObjectId`
 is a 12-byte unique identifiers suitable for use as the value of an
 ``_id`` field.