error:
       … while calling the 'toJSON' builtin
         at /pwd/in.nix:1:1:
            1| builtins.toJSON (builtins.substring 0 1 "𝒫")
             | ^
            2|

       error: JSON processing error: [json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xF0