diff --git a/source/core/sharding-shard-key.txt b/source/core/sharding-shard-key.txt
index be88e8a08b6..85403f69c32 100644
--- a/source/core/sharding-shard-key.txt
+++ b/source/core/sharding-shard-key.txt
@@ -91,16 +91,20 @@ recreating an index on just the shard key.
 Unique Indexes
 ~~~~~~~~~~~~~~
 
-For a sharded collection, only the ``_id`` field index and the index on
-the shard key or a :term:`compound index` where the shard key is a
-:ref:`prefix <compound-index-prefix>` can be :doc:`unique
-</core/index-unique>`:
+For a sharded collection, only the index on the shard key or a :term:`compound
+index` where the shard key is a :ref:`prefix <compound-index-prefix>` can be
+:doc:`unique </core/index-unique>`:
 
 - You cannot shard a collection that has unique indexes on other fields.
 
 - You cannot create unique indexes on other fields for a sharded
   collection.
 
+.. important::
+
+    The ``_id`` field index can only be :doc:`unique </core/index-unique>`
+    across the sharded cluster if it is the shard key.
+
 Through the use of the unique index on the shard key, MongoDB *can*
 enforce uniqueness on the shard key values. MongoDB enforces uniqueness
 on the *entire* key combination, and not individual components of the