error:
       … while calling the 'toJSON' builtin
         at /pwd/lang/eval-fail-toJSON-non-utf-8.nix:1:1:
            1| builtins.toJSON "_invalid UTF-8: ÿ_"
             | ^
            2|

       error: JSON serialization error: [json.exception.type_error.316] invalid UTF-8 byte at index 16: 0xFF