diff --git a/src/ra_server.erl b/src/ra_server.erl
index 8f96d52f..54ec223f 100644
--- a/src/ra_server.erl
+++ b/src/ra_server.erl
@@ -462,6 +462,13 @@ handle_leader({PeerId, #append_entries_reply{term = Term}},
                     [LogId, PeerId, Term, CurTerm]),
             {follower, update_term(Term, State0#{leader_id => undefined}), []}
     end;
+handle_leader({PeerId, #append_entries_reply{success = false}},
+              State0 = #{cfg := #cfg{log_id = LogId},
+                         cluster := Nodes})
+  when not is_map_key(PeerId, Nodes) ->
+    ?WARN("~ts: saw append_entries_reply from unknown peer ~w",
+          [LogId, PeerId]),
+    {leader, State0, []};
 handle_leader({PeerId, #append_entries_reply{success = false,
                                              next_index = NextIdx,
                                              last_index = LastIdx,
diff --git a/test/ra_server_SUITE.erl b/test/ra_server_SUITE.erl
index 64685dbf..24808907 100644
--- a/test/ra_server_SUITE.erl
+++ b/test/ra_server_SUITE.erl
@@ -23,6 +23,7 @@ all() ->
      append_entries_reply_success_promotes_nonvoter,
      append_entries_reply_success,
      append_entries_reply_no_success,
+     append_entries_reply_no_success_from_unknown_peer,
      follower_request_vote,
      follower_pre_vote,
      pre_vote_receives_pre_vote,
@@ -974,6 +975,21 @@ append_entries_reply_no_success(_Config) ->
       ]} = ra_server:handle_leader(Msg, State),
     ok.
 
+append_entries_reply_no_success_from_unknown_peer(_Config) ->
+    N1 = ?N1, N2 = ?N2,
+    Cluster = #{N1 => new_peer()},
+    State = (base_state(3, ?FUNCTION_NAME))#{commit_index => 1,
+                             last_applied => 1,
+                             cluster => Cluster,
+                             machine_state => <<"hi1">>},
+    % n2 is an unknown peer
+    Msg = {N2, #append_entries_reply{term = 5, success = false, next_index = 2,
+                                     last_index = 1, last_term = 1}},
+    % The reply from n2 is ignored
+    {leader, State, []} = ra_server:handle_leader(Msg, State),
+
+    ok.
+
 follower_request_vote(_Config) ->
     N2 = ?N2, N3 = ?N3,
     State = base_state(3, ?FUNCTION_NAME),