diff --git a/sw/lib/ocaml/mapCanvas.ml b/sw/lib/ocaml/mapCanvas.ml
index e51a2035a3..8a9fb5f02d 100644
--- a/sw/lib/ocaml/mapCanvas.ml
+++ b/sw/lib/ocaml/mapCanvas.ml
@@ -176,23 +176,26 @@ class basic_widget = fun ?(height=800) ?width ?(projection = Mercator) ?georef (
     method world_of = fun wgs84 ->
       match georef with
 	Some georef -> begin
-	  match projection with
-	    UTM ->
-	      let utmref = LL.utm_of LL.WGS84 georef
-	      and utm = LL.utm_of LL.WGS84 wgs84 in
-	      let (wx, y) = LL.utm_sub utm utmref in
-	      (wx, -.y)
-	  | Mercator ->
-	      let mlref = LL.mercator_lat georef.LL.posn_lat
-	      and ml = LL.mercator_lat wgs84.LL.posn_lat in
-	      let xw = (wgs84.LL.posn_long -. georef.LL.posn_long) *. mercator_coeff
-	      and yw = -. (ml -. mlref) *. mercator_coeff in
-	      (xw, yw)
-	  | LambertIIe ->
-	      let lbtref = LL.lambertIIe_of georef
-	      and lbt = LL.lambertIIe_of wgs84 in
-	      let (wx, y) = LL.lbt_sub lbt lbtref in
-	      (wx, -.y)
+	  try
+	    match projection with
+	      UTM ->
+		let utmref = LL.utm_of LL.WGS84 georef
+		and utm = LL.utm_of LL.WGS84 wgs84 in
+		let (wx, y) = LL.utm_sub utm utmref in
+		(wx, -.y)
+	    | Mercator ->
+		let mlref = LL.mercator_lat georef.LL.posn_lat
+		and ml = LL.mercator_lat wgs84.LL.posn_lat in
+		let xw = (wgs84.LL.posn_long -. georef.LL.posn_long) *. mercator_coeff
+		and yw = -. (ml -. mlref) *. mercator_coeff in
+		(xw, yw)
+	    | LambertIIe ->
+		let lbtref = LL.lambertIIe_of georef
+		and lbt = LL.lambertIIe_of wgs84 in
+		let (wx, y) = LL.lbt_sub lbt lbtref in
+		(wx, -.y)
+	  with
+	    _ -> (0., 0.) (** Don't want to break everything with bad coordinates *)
 	end
       | None -> failwith "#world_of : no georef"
 
diff --git a/sw/lib/ocaml/pprz.ml b/sw/lib/ocaml/pprz.ml
index dc9fc7c5ee..785c8bbd1b 100644
--- a/sw/lib/ocaml/pprz.ml
+++ b/sw/lib/ocaml/pprz.ml
@@ -393,6 +393,13 @@ module Messages(Class:CLASS) = struct
     | [] -> invalid_arg (sprintf "Pprz.values_of_string: %s" s)
 
   let string_of_message = fun msg values ->
+    (** Check that the values are compatible with this message *)
+    List.iter 
+      (fun (k, _) ->
+	if not (List.mem_assoc k msg.fields) 
+	then invalid_arg (sprintf "Pprz.string_of_message: unknown field '%s' in message '%s'" k msg.name))
+      values;
+
     String.concat " "
       (msg.name::
        List.map