Does not return a valid JSON string in certain cases about fast-stable-stringify HOT 8 CLOSED

const Stringify = require('json-stable-stringify');
const fastString = require('fast-stable-stringify');

const obj = {
        a: {
            e: {
                alpha: 'feed5c9cdae3cae69202a5af67fc99c5dc9e8d30e1aef8d294f6bfb0c2aad45a',
                beta: '4def11058e66f2d8824b1c1b54e5f6efe0537338c32be558ce4e1f8ce6b66893',
                theta: '9bcb68bad9e70e17ddebca3a6c668af59b13fd2e06f558ea00c259113bafda8c',
                phi: 'd50a4ace00559b04fe99f117cf942097f4293652ca4454492e67e2e36dccedea',
                omega: '111ce70909455ee61f8c53ff18701c557714972856708277a2d1ac9d7a7d7a13',
                delta: '66cee05cd51948b4ab02c1393fae28c2d557f92f21275ce1ca2ac69fff54b830'
            },
             f: {
                cat: true,
                dog: false,
                lizard: true,
                bird: false,
                fish: false,
                bug: false
            },
             secretAgent: 'For queen and country',
             james: 'His name',
             bond: 'His last name'
         },
        b: {
            g: { age: -6 }, f: {}
        },
        c: {
            h: {
                door: 'knob',
                window: 'sill',
                hallway: undefined
            },
             f: {},
             hdoor: 'knob',
             hwindow: 'sill',
             hhallway: undefined },
        d:{
             i: 0.69,
             j: 'english',
             k: '32bit',
             l: 8,
             m: '81448148184148141908',
             n: '2.1875',
             o: '10,true,true',
             p: 'hello world',
             q: 4,
             r: 1,
             s: 3,
             t: 'operating systems',
             u: 'versions'
         }
  };
console.log( Stringify(obj) === fastString(obj) );
// the library produces invalid JSON string.
// meanwhile json-stable-stringify produces a valid JSON string