diff --git a/akka-docs/java/routing.rst b/akka-docs/java/routing.rst
index 110c5d3944..4d01642a72 100644
--- a/akka-docs/java/routing.rst
+++ b/akka-docs/java/routing.rst
@@ -339,6 +339,19 @@ If you are interested in how to use the VoteCountRouter it looks like this:
 
 .. includecode:: code/akka/docs/jrouting/CustomRouterDocTestBase.java#crTest
 
+.. caution::
+
+   When creating a cutom router the resulting RoutedActorRef optimizes the
+   sending of the message so that it does NOT go through the router’s mailbox
+   unless the route returns an empty recipient set.
+
+   This means that the ``route`` function defined in the ``RouterConfig``
+   or the function returned from ``CreateCustomRoute`` in
+   ``CustomRouterConfig`` is evaluated concurrently without protection by
+   the RoutedActorRef: either provide a reentrant (i.e. pure) implementation
+   or do the locking yourself!
+
+
 Configured Custom Router
 ************************
 
diff --git a/akka-docs/scala/routing.rst b/akka-docs/scala/routing.rst
index 22fac9c226..737c9e31e7 100644
--- a/akka-docs/scala/routing.rst
+++ b/akka-docs/scala/routing.rst
@@ -339,6 +339,19 @@ All in all the custom router looks like this:
 If you are interested in how to use the VoteCountRouter you can have a look at the test class
 `RoutingSpec <https://github.com/akka/akka/blob/master/akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala>`_
 
+.. caution::
+
+   When creating a cutom router the resulting RoutedActorRef optimizes the
+   sending of the message so that it does NOT go through the router’s mailbox
+   unless the route returns an empty recipient set.
+
+   This means that the ``route`` function defined in the ``RouterConfig``
+   or the function returned from ``CreateCustomRoute`` in
+   ``CustomRouterConfig`` is evaluated concurrently without protection by
+   the RoutedActorRef: either provide a reentrant (i.e. pure) implementation
+   or do the locking yourself!
+
+
 Configured Custom Router
 ************************