Chai/Node.js: Expect(value).to.be.NaN issue

I have these form fields I need to validate with chai expect. They all work (I get errors) except for the fields that require characters - expect(value).to.be.NaN is not working... The code:

var inputs = [response.fname, response.surname,
  response.age, response.password
];

for (var i = 0; i < inputs.length; i++) {
  var stackErrors = validate(i);
  console.log(stackErrors);
}

function validate(i) {
  var fields = inputs[i];

  try {
    if (i == 0) {
      expect(fields).to.be.Nan;
    }

    if (i == 1) {
      expect(fields).to.be.Nan;
    }

    if (i == 2) {
      expect(fields).not.to.be.NaN;
    }

    if (i == 3) {
      expect(fields).to.have.length.above(6);
    }

    return "OK";
  } catch (error) {
    console.log(error);
    return error;
  }
}

But if test the field like in the example bellow, right after the inputs array, I get the expected assertion error...:

expect(inputs[0]).to.be.NaN;

Why is this happening? Thanks in advance

Thanks to the hint of @Lazyexpert (I've noticed you have Nan in your code, it is not the same as NaN.), I found the obvious solution...(a case sensitive issue, actualy)

try {
    if (i == 0) {
      expect(fields).to.be.NaN; //instead of Nan...
    }