diff --git a/src/core/CCString.ml b/src/core/CCString.ml
index 6c5ae1c5..3b837ab0 100644
--- a/src/core/CCString.ml
+++ b/src/core/CCString.ml
@@ -1173,9 +1173,11 @@ let of_hex s = try Some (of_hex_exn s) with Invalid_argument _ -> None
 *)
 
 (*$Q
-    Q.(string) (fun s -> \
-        of_hex_exn (to_hex s) = s)
-    *)
+  Q.(string) (fun s -> \
+    of_hex_exn (to_hex s) = s)
+  Q.(string) (fun s -> \
+    String.for_all (function 'A'..'F'|'a'..'f'|'0'..'9' -> true |  _ -> false) @@ to_hex s)
+*)
 
 let pp_buf buf s =
   Buffer.add_char buf '"';