diff --git a/big_tests/tests/xep_0352_csi_SUITE.erl b/big_tests/tests/xep_0352_csi_SUITE.erl
index 3b0381422e..48b931eb36 100644
--- a/big_tests/tests/xep_0352_csi_SUITE.erl
+++ b/big_tests/tests/xep_0352_csi_SUITE.erl
@@ -6,6 +6,7 @@
 -compile([export_all, nowarn_export_all]).
 
 -import(domain_helper, [host_type/0]).
+-import(config_parser_helper, [default_mod_config/1, mod_config/2]).
 
 -define(CSI_BUFFER_MAX, 10).
 
@@ -35,8 +36,8 @@ suite() ->
 init_per_suite(Config) ->
     NewConfig = dynamic_modules:save_modules(host_type(), Config),
     dynamic_modules:ensure_modules(
-      host_type(), [{mod_offline, config_parser_helper:mod_config(mod_offline, #{})},
-                    {mod_csi, [{buffer_max, ?CSI_BUFFER_MAX}]}]),
+      host_type(), [{mod_offline, default_mod_config(mod_offline)},
+                    {mod_csi, mod_config(mod_csi, #{buffer_max => ?CSI_BUFFER_MAX})}]),
     [{escalus_user_db, {module, escalus_ejabberd}} | escalus:init_per_suite(NewConfig)].
 
 end_per_suite(Config) ->
diff --git a/src/ejabberd_c2s.erl b/src/ejabberd_c2s.erl
index 5ffc727eed..291cbca3bf 100644
--- a/src/ejabberd_c2s.erl
+++ b/src/ejabberd_c2s.erl
@@ -2740,8 +2740,7 @@ maybe_csi_inactive_optimisation(Acc, {From,To,El}, #state{csi_buffer = Buffer} =
     {ok, Acc, NewState}.
 
 flush_or_buffer_packets(State) ->
-    MaxBuffSize = gen_mod:get_module_opt(State#state.host_type, mod_csi,
-                                         buffer_max, 20),
+    MaxBuffSize = gen_mod:get_module_opt(State#state.host_type, mod_csi, buffer_max),
     case length(State#state.csi_buffer) > MaxBuffSize of
         true ->
             flush_csi_buffer(State);
diff --git a/src/mod_csi.erl b/src/mod_csi.erl
index 59a8b1fe48..f305d6fe23 100644
--- a/src/mod_csi.erl
+++ b/src/mod_csi.erl
@@ -51,8 +51,10 @@ ensure_metrics(HostType) ->
 config_spec() ->
     #section{
        items = #{<<"buffer_max">> => #option{type = int_or_infinity,
-                                             validate = non_negative}}
-      }.
+                                             validate = non_negative}},
+       format_items = map,
+       defaults = #{<<"buffer_max">> => 20}
+    }.
 
 -spec supported_features() -> [atom()].
 supported_features() ->
diff --git a/test/common/config_parser_helper.erl b/test/common/config_parser_helper.erl
index 89270730f1..2b214e00f2 100644
--- a/test/common/config_parser_helper.erl
+++ b/test/common/config_parser_helper.erl
@@ -538,7 +538,7 @@ all_modules() ->
                           port => 2222, transport => <<"tcp">>, type => stun,
                           username => <<"username">>},
                         #{host => <<"192.168.0.1">>, type => turn}]},
-      mod_csi => [{buffer_max, 40}],
+      mod_csi => mod_config(mod_csi, #{buffer_max => 40}),
       mod_muc_log =>
           mod_config(mod_muc_log,
                      #{access_log => muc,
@@ -855,6 +855,8 @@ default_mod_config(mod_cache_users) ->
 default_mod_config(mod_caps) ->
     #{cache_size => 1000,
       cache_life_time => timer:hours(24) div 1000};
+default_mod_config(mod_csi) ->
+    #{buffer_max => 20};
 default_mod_config(mod_disco) ->
     #{extra_domains => [], server_info => [],
       users_can_see_hidden_services => true, iqdisc => one_queue};
diff --git a/test/config_parser_SUITE.erl b/test/config_parser_SUITE.erl
index 96fae27c88..62a4e4d0e8 100644
--- a/test/config_parser_SUITE.erl
+++ b/test/config_parser_SUITE.erl
@@ -1656,10 +1656,11 @@ mod_carboncopy(_Config) ->
     check_iqdisc(mod_carboncopy).
 
 mod_csi(_Config) ->
+    check_module_defaults(mod_csi),
     T = fun(K, V) -> #{<<"modules">> => #{<<"mod_csi">> => #{K => V}}} end,
-    M = fun(K, V) -> modopts(mod_csi, [{K, V}]) end,
-    ?cfgh(M(buffer_max, 10), T(<<"buffer_max">>, 10)),
-    ?cfgh(M(buffer_max, infinity), T(<<"buffer_max">>, <<"infinity">>)),
+    P = [modules, mod_csi],
+    ?cfgh(P ++ [buffer_max], 10, T(<<"buffer_max">>, 10)),
+    ?cfgh(P ++ [buffer_max], infinity, T(<<"buffer_max">>, <<"infinity">>)),
     ?errh(T(<<"buffer_max">>, -1)).
 
 mod_disco(_Config) ->