diff --git a/docs/ref/settings.txt b/docs/ref/settings.txt
index e4cea3ddef..a61612f49b 100644
--- a/docs/ref/settings.txt
+++ b/docs/ref/settings.txt
@@ -839,7 +839,9 @@ but for all apps.
 Default: ``None``
 
 The alias of the database that this database should mirror during
-testing.
+testing. It depends on transactions and therefore must be used within
+:class:`~django.test.TransactionTestCase` instead of
+:class:`~django.test.TestCase`.
 
 This setting exists to allow for testing of primary/replica
 (referred to as master/slave by some databases)
diff --git a/docs/topics/testing/advanced.txt b/docs/topics/testing/advanced.txt
index e6ebc346dd..33d2836eb9 100644
--- a/docs/topics/testing/advanced.txt
+++ b/docs/topics/testing/advanced.txt
@@ -230,7 +230,9 @@ will *not* be created. Instead the connection to ``replica``
 will be redirected to point at ``default``. As a result, writes to
 ``default`` will appear on ``replica`` -- but because they are actually
 the same database, not because there is data replication between the
-two databases.
+two databases. As this depends on transactions, the tests must use
+:class:`~django.test.TransactionTestCase` instead of
+:class:`~django.test.TestCase`.
 
 .. _topics-testing-creation-dependencies: