diff --git a/src/Tiny_httpd_util.ml b/src/Tiny_httpd_util.ml
index f2b769e2..370384f6 100644
--- a/src/Tiny_httpd_util.ml
+++ b/src/Tiny_httpd_util.ml
@@ -1,7 +1,7 @@
 
 (* test utils *)
 (*$inject
-  let pp_res f = function Ok x -> f x | Error e -> Printexc.to_string e
+  let pp_res f = function Ok x -> f x | Error e -> e
   let pp_res_query = (Q.Print.(pp_res (list (pair string string))))
   let err_map f = function Ok x-> Ok (f x) | Error e -> Error e
   let sort_l l = List.sort compare l
@@ -17,7 +17,7 @@ let percent_encode ?(skip=fun _->false) s =
       | (' ' | '!' | '"' | '#' | '$' | '%' | '&' | '\'' | '(' | ')' | '*' | '+'
         | ',' | '/' | ':' | ';' | '=' | '?' | '@' | '[' | ']' | '~')
         as c ->
-        Printf.bprintf buf "%%%x" (Char.code c)
+        Printf.bprintf buf "%%%X" (Char.code c)
       | c -> Buffer.add_char buf c)
     s;
   Buffer.contents buf
@@ -25,7 +25,11 @@ let percent_encode ?(skip=fun _->false) s =
 (*$= & ~printer:(fun s->s)
   "hello%20world" (percent_encode "hello world")
   "%23%25^%24%40^%40" (percent_encode "#%^$@^@")
-  "a%20ohm%2b5235%25%26%40%23%20---%20_" (percent_encode "a ohm+5235%&@# --- _")
+  "a%20ohm%2B5235%25%26%40%23%20---%20_" (percent_encode "a ohm+5235%&@# --- _")
+*)
+
+(*$= & ~printer:Q.(Print.(option string))
+  (Some "?") (percent_decode @@ percent_encode "?")
 *)
 
 let hex_int (s:string) : int = Scanf.sscanf s "%x" (fun x->x)
@@ -60,16 +64,21 @@ let percent_decode (s:string) : _ option =
         | None -> Q.Test.fail_report "invalid percent encoding")
 *)
 
-let parse_query s : (_ list, _) result=
+exception Invalid_query
+
+let parse_query s : (_ list, string) result=
   let pairs = ref [] in
   let is_sep_ = function '&' | ';' -> true | _ -> false in
+  let i = ref 0 in
+  let j = ref 0 in
   try
-    let i = ref 0 in
-    let j = ref 0 in
+    let percent_decode s =
+      match percent_decode s with Some x -> x | None -> raise Invalid_query
+    in
     let parse_pair () =
       let eq = String.index_from s !i '=' in
-      let k = String.sub s !i (eq- !i) in
-      let v = String.sub s (eq+1) (!j-eq-1) in
+      let k = percent_decode @@ String.sub s !i (eq- !i) in
+      let v = percent_decode @@ String.sub s (eq+1) (!j-eq-1) in
       pairs := (k,v) :: !pairs;
     in
     while !i < String.length s do
@@ -85,7 +94,10 @@ let parse_query s : (_ list, _) result=
       )
     done;
     Ok !pairs
-  with e -> Error e
+  with
+  | Invalid_argument _ | Not_found | Failure _ ->
+    Error (Printf.sprintf "error in parse_query for %S: i=%d,j=%d" s !i !j)
+  | Invalid_query -> Error ("invalid query string: " ^ s)
 
 (*$= & ~printer:pp_res_query ~cmp:eq_sorted
   (Ok ["a", "b"; "c", "d"]) (parse_query "a=b&c=d")
diff --git a/src/Tiny_httpd_util.mli b/src/Tiny_httpd_util.mli
index dada640b..32de8908 100644
--- a/src/Tiny_httpd_util.mli
+++ b/src/Tiny_httpd_util.mli
@@ -13,7 +13,7 @@ val percent_decode : string -> string option
 (** Inverse operation of {!percent_encode}.
     Can fail since some strings are not valid percent encodings. *)
 
-val parse_query : string -> ((string*string) list, exn) result
+val parse_query : string -> ((string*string) list, string) result
 (** Parse a query as a list of ['&'] or [';'] separated [key=value] pairs.
     The order might not be preserved.
     @since 0.3