diff --git a/big_tests/tests/xep_0352_csi_SUITE.erl b/big_tests/tests/xep_0352_csi_SUITE.erl
index 753a84de175..a991a1be68b 100644
--- a/big_tests/tests/xep_0352_csi_SUITE.erl
+++ b/big_tests/tests/xep_0352_csi_SUITE.erl
@@ -28,7 +28,8 @@ all_tests() ->
      alice_gets_buffered_messages_after_reconnection_with_sm,
      alice_gets_buffered_messages_after_stream_resumption,
      bob_gets_msgs_from_inactive_alice,
-     alice_is_inactive_but_sends_sm_req_and_recives_ack
+     alice_is_inactive_but_sends_sm_req_and_recives_ack,
+     invalid_csi_request_returns_error
     ].
 
 suite() ->
@@ -65,6 +66,13 @@ server_announces_csi(Config) ->
     {ok, _Client, Features} = escalus_connection:start(Spec, Steps),
     true = proplists:get_value(client_state_indication, Features).
 
+invalid_csi_request_returns_error(Config) ->
+    escalus:fresh_story(Config, [{alice, 1}], fun(Alice) ->
+        escalus:send(Alice, csi_stanza(<<"invalid">>)),
+        Stanza = escalus:wait_for_stanza(Alice),
+        escalus:assert(is_error, [<<"modify">>, <<"bad-request">>], Stanza)
+    end).
+
 alice_is_inactive_and_no_stanza_arrived(Config) ->
     escalus:fresh_story(Config, [{alice, 1}, {bob, 1}], fun(Alice, Bob) ->
         given_client_is_inactive_and_no_messages_arrive(Alice),
diff --git a/src/mod_csi.erl b/src/mod_csi.erl
index f53d0acf2db..d84e35a409e 100644
--- a/src/mod_csi.erl
+++ b/src/mod_csi.erl
@@ -147,7 +147,12 @@ reroute_unacked_messages(Acc, #{c2s_data := C2SData}, _) ->
 handle_csi_request(Acc, Params, #xmlel{name = <<"inactive">>}) ->
     handle_inactive_request(Acc, Params);
 handle_csi_request(Acc, Params, #xmlel{name = <<"active">>}) ->
-    handle_active_request(Acc, Params).
+    handle_active_request(Acc, Params);
+handle_csi_request(Acc, _, _) ->
+    {From, To, El} = mongoose_acc:packet(Acc),
+    Error = jlib:make_error_reply(El, mongoose_xmpp_errors:bad_request()),
+    ErrorAcc = mongoose_acc:update_stanza(#{from_jid => From, to_jid => To, element => Error }, Acc),
+    mongoose_c2s_acc:to_acc(Acc, route, ErrorAcc).
 
 -spec handle_inactive_request(mongoose_acc:t(), mongoose_c2s_hooks:params()) -> mongoose_acc:t().
 handle_inactive_request(Acc, #{c2s_data := C2SData} = _Params) ->