diff --git a/lib/server/connection.ml b/lib/server/connection.ml
index 4daf779..997ea28 100644
--- a/lib/server/connection.ml
+++ b/lib/server/connection.ml
@@ -34,6 +34,10 @@ type reply = string * string list
 
 type 'a result = ('a, reply) Result.t
 
+let list_of_errors = function
+  | Ok () -> []
+  | Error e -> [e]
+
 let reply t (num, params) =
   let prefix = Server_info.prefix t.server_info in
   let target =
@@ -59,6 +63,7 @@ let unknowncommand cmd    = "421", [cmd; "Unknown command"]
 let nonicknamegiven       = "431", ["No nickname given"]
 let erroneusnickname nick = "432", [nick; "Erroneus nickname"]
 let nicknameinuse nick    = "433", [nick; "Nickname is already in use"]
+let usernotinchannel n c  = "442", [n; c; "They aren't on that channel"]
 let notonchannel chan     = "442", [chan; "You're not on that channel"]
 let notregistered         = "451", ["You have not registered"]
 let needmoreparams cmd    = "461", [cmd; "Not enough parameters"]
@@ -215,25 +220,29 @@ let on_set_chan_mode chan me modestr args ~router =
   set_chan_mode chan ~from:me ~add:chg.chan_modes.add ~rem:chg.chan_modes.rem;
   Option.iter (set_chan_key chan ~from:me) chg.chan_key;
   Option.iter (set_chan_limit chan ~from:me) chg.chan_limit;
-  List.iter
-    (fun (op, mode, nick) ->
-       try
-         let user = Router.find_user router nick in
-         let mem = Router.membership chan user in
-         let priv = match mode with `o -> Router.Operator | `v -> Voice in
-         match op with
-         | `add ->
-           set_member_priv mem priv ~from:me
-         | `rem ->
-           if mem.mem_priv = priv then
-             set_member_priv mem Normal ~from:me
-       with Not_found ->
-         ())
-    chg.chan_privs;
 
-  (* TODO: ban (+b) *)
+  (* TODO: MODE <chan> +b *)
 
-  Ok []
+  let results =
+    List.map
+      (fun (dir, mode, nick) ->
+         let* user = try Ok (Router.find_user router nick)
+           with Not_found -> Error (nosuchnick nick) in
+         let* mem = try Ok (Router.membership chan user)
+           with Not_found -> Error (usernotinchannel (User.nick user) (Chan.name chan)) in
+         let priv : Router.priv = match mode with
+           | `o -> Operator
+           | `v -> Voice
+         in
+         begin match dir with
+           | `add -> set_member_priv mem priv ~from:me
+           | `rem -> if mem.mem_priv = priv then set_member_priv mem Normal ~from:me
+         end;
+         Ok ())
+      chg.chan_privs
+  in
+
+  Ok (List.flat_map list_of_errors results)
 
 let on_msg_mode t name args =
   let* me = require_registered t in
@@ -636,10 +645,14 @@ let pp_args ppf (cmd, params) =
   Fmt.pf ppf "@[%s@ %a@]" cmd (Fmt.list (Fmt.fmt "%S") ~sep:Fmt.sp) params
 
 let on_msg t (msg : Msg.t) : unit =
-  split_command_params msg.command msg.params |>
-  List.iter
-    (fun args ->
-       trace (fun m -> m "@[%a:@ %a@]" pp_sockaddr t.addr pp_args args);
-       match dispatch t args with
-       | Ok () -> ()
-       | Error err -> reply t err)
+  let results =
+    List.map
+      (fun cmd ->
+         trace (fun m -> m "@[%a:@ %a@]" pp_sockaddr t.addr pp_args cmd);
+         dispatch t cmd)
+      (split_command_params
+         msg.command
+         msg.params)
+  in
+  List.iter (reply t)
+    (List.flat_map list_of_errors results)