diff --git a/conf/messages.xml b/conf/messages.xml
index ab5566b83d..29a1b1b0c5 100644
--- a/conf/messages.xml
+++ b/conf/messages.xml
@@ -450,6 +450,10 @@
       <field name="level" type="string" values="CATASTROPHIC|CRITIC|WARNING"/> 
       <field name="value" type="float"/>
   </message>
+   <message name="AIR_PROX" ID="2">
+      <field name="ac_id" type="string"/>
+      <field name="level" type="string" values="CATASTROPHIC|CRITIC|WARNING"/>
+  </message>
 </class>
 
 <class name="datalink">
diff --git a/sw/ground_segment/tmtc/server.ml b/sw/ground_segment/tmtc/server.ml
index 5cad1b1539..383ac97ec6 100644
--- a/sw/ground_segment/tmtc/server.ml
+++ b/sw/ground_segment/tmtc/server.ml
@@ -256,8 +256,8 @@ let ac_msg = fun log ac_name a m ->
     let msg = Tele_Pprz.message_of_id msg_id in
     log_and_parse log ac_name a msg values
   with
-    Pprz.Unknown_msg_name x ->
-      fprintf stderr "Unknown message %s from %s: %s\n" x ac_name m
+    Pprz.Unknown_msg_name (x, c) ->
+      fprintf stderr "Unknown message %s in class %s from %s: %s\n" x c ac_name m
   | x -> prerr_endline (Printexc.to_string x)
 
 
@@ -464,12 +464,17 @@ let periodic_airprox_check = fun name ->
   let list_ac = List.map (fun name -> Hashtbl.find aircrafts name) ac_names in
 
   let check_airprox = fun ac ->
-    match Airprox.check_airprox thisac ac with
-      None -> ()
-    | Some level ->
-	let vs =
-	  ["ac_id", Pprz.String (thisac.id ^ "," ^ ac.id) ; "level", Pprz.String level; "value", Pprz.String ac.id] in
-	Alerts_Pprz.message_send my_id "AIR_PROX" vs in
+    try
+      match Airprox.check_airprox thisac ac with
+	None -> ()
+      | Some level ->
+	  let vs =
+	    ["ac_id", Pprz.String (thisac.id ^ "," ^ ac.id) ; "level", Pprz.String level] in
+	  Alerts_Pprz.message_send my_id "AIR_PROX" vs
+    with
+      x -> fprintf stderr "check_airprox: %s\n%!" (Printexc.to_string x)
+
+ in
   
   List.iter 
     (fun ac ->
diff --git a/sw/lib/ocaml/pprz.ml b/sw/lib/ocaml/pprz.ml
index 764ad558b6..49d9dad39d 100644
--- a/sw/lib/ocaml/pprz.ml
+++ b/sw/lib/ocaml/pprz.ml
@@ -209,7 +209,7 @@ let sprint_value = fun buf i field_type v ->
 
 module type CLASS = sig val name : string end
 
-exception Unknown_msg_name of string
+exception Unknown_msg_name of string * string
 
 module Protocol(Class:CLASS) = struct
   let stx = Char.chr 0x05
@@ -222,7 +222,11 @@ module Protocol(Class:CLASS) = struct
     with
       Not_found -> failwith (sprintf "Unknown message class: %s" Class.name)
   let message_of_id = fun id -> Hashtbl.find messages_by_id id
-  let message_of_name = fun name -> Hashtbl.find messages_by_name name
+  let message_of_name = fun name ->
+    try
+      Hashtbl.find messages_by_name name
+    with
+      Not_found -> raise (Unknown_msg_name (name, Class.name))
 
   let length = fun buf start ->
     let len = String.length buf - start in
@@ -289,8 +293,7 @@ module Protocol(Class:CLASS) = struct
 	    let values = List.map2 (fun (field_name, field) v -> (field_name, value field._type v)) msg.fields args in
 	    (msg_id, values)
 	  with
-	    Not_found -> raise (Unknown_msg_name msg_name)
-	  | Invalid_argument "List.map2" -> failwith (sprintf "Pprz.values_of_string: '%s'" s)
+	    Invalid_argument "List.map2" -> failwith (sprintf "Pprz.values_of_string: '%s'" s)
 	end
     | [] -> invalid_arg "Pprz.values_of_string"
 
diff --git a/sw/lib/ocaml/pprz.mli b/sw/lib/ocaml/pprz.mli
index c92ccd1732..1cd7c9038f 100644
--- a/sw/lib/ocaml/pprz.mli
+++ b/sw/lib/ocaml/pprz.mli
@@ -55,7 +55,9 @@ val int_assoc : string -> values -> int
 val int32_assoc : string -> values -> int
 (** May raise Not_found or Invalid_argument *)
 
-exception Unknown_msg_name of string
+exception Unknown_msg_name of string * string
+(** [Unknown_msg_name (name, class_name)] Raised if message [name] is not
+found in class [class_name]. *)
 
 module type CLASS = sig val name : string end
 module Protocol : functor (Class : CLASS) -> sig