diff --git a/vendors/ocplib-json-typed/lib_json_typed/json_repr.ml b/vendors/ocplib-json-typed/lib_json_typed/json_repr.ml
index 8f7b003a4..12a235034 100644
--- a/vendors/ocplib-json-typed/lib_json_typed/json_repr.ml
+++ b/vendors/ocplib-json-typed/lib_json_typed/json_repr.ml
@@ -128,6 +128,7 @@ let pp_string ppf s =
     | '\b' -> Format.fprintf ppf "\\b"
     | '\t' -> Format.fprintf ppf "\\t"
     | '\\' -> Format.fprintf ppf "\\\\"
+    | '\x00' .. '\x1F' as c -> Format.fprintf ppf "\\u%04x" (Char.code c)
     | c -> Format.fprintf ppf "%c" c
   done ;
   Format.fprintf ppf "\""