Why is moment.js diff method returning NaN?

Terminal output:

Now:  { _d: Sat Jan 13 2018 02:39:25 GMT-0400 (AST),
      _isUTC: false,
      _a: null,
      _lang: false }
Expiration Date: { _d: Wed Feb 13 2013 02:00:15 GMT-0400 (AST),
      _isUTC: false,
      _a: null,
      _lang: false }
Difference between Now and Expiration Date: NaN

Code:

console.log('Difference between Now and Expiration Date:', now.diff(expDate, 'months', true));

moment.js source:

diff : function (input, val, asFloat) {
            var inputMoment = this._isUTC ? moment(input).utc() : moment(input).local(),
                zoneDiff = (this.zone() - inputMoment.zone()) * 6e4,
                diff = this._d - inputMoment._d - zoneDiff,
                year = this.year() - inputMoment.year(),
                month = this.month() - inputMoment.month(),
                date = this.date() - inputMoment.date(),
                output;
            if (val === 'months') {
                output = year * 12 + month + date / 30;
            } else if (val === 'years') {
                output = year + (month + date / 30) / 12;
            } else {
                output = val === 'seconds' ? diff / 1e3 : // 1000
                    val === 'minutes' ? diff / 6e4 : // 1000 * 60
                    val === 'hours' ? diff / 36e5 : // 1000 * 60 * 60
                    val === 'days' ? diff / 864e5 : // 1000 * 60 * 60 * 24
                    val === 'weeks' ? diff / 6048e5 : // 1000 * 60 * 60 * 24 * 7
                    diff;
            }
            return asFloat ? output : round(output);
        }