diff --git a/src/ra_machine.erl b/src/ra_machine.erl
index ba54092a..3213272d 100644
--- a/src/ra_machine.erl
+++ b/src/ra_machine.erl
@@ -232,6 +232,20 @@
 
 -callback init_aux(Name :: atom()) -> term().
 
+-callback handle_aux(ra_server:ra_state(),
+                     {call, From :: from()} | cast,
+                     Command :: term(),
+                     AuxState,
+                     State) ->
+    {reply, Reply :: term(), AuxState, State} |
+    {reply, Reply :: term(), AuxState, State,
+     [{monitor, process, aux, pid()}]} |
+    {no_reply, AuxState, State} |
+    {no_reply, AuxState, State,
+     [{monitor, process, aux, pid()}]}
+      when AuxState :: term(),
+           State :: ra_aux:state().
+
 -callback handle_aux(ra_server:ra_state(),
                      {call, From :: from()} | cast,
                      Command :: term(),