diff --git a/utils/warnings.ml b/utils/warnings.ml
index 70cd0589e..05aeb63ae 100644
--- a/utils/warnings.ml
+++ b/utils/warnings.ml
@@ -68,20 +68,15 @@ let letter = function        (* 'a' is all *)
   | Unused_var_strict _ ->      'z'
 ;;
 
-let check c =
-  try ignore (String.index "acdeflmpsuvxyzACDEFLMPSUVXYZ" c)
-  with _ -> raise (Arg.Bad (Printf.sprintf "unknown warning option %c" c))
-;;    
-
-let active = Array.create 26 true;;
-let error = Array.create 26 false;;
+let active = Array.create 27 true;;
+let error = Array.create 27 false;;
 
 let translate c =
-  check c;
   if c >= 'A' && c <= 'Z' then
     (Char.code c - Char.code 'A', true)
-  else
+  else if c >= 'a' && c <= 'z' then
     (Char.code c - Char.code 'a', false)
+  else (26, false)
 ;;
 
 let is_active x =