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, 
            response.email, response.phone, 
            response.city, response.country, 
            response.postal_code];


        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);
             }

            if (i == 4) {
                expect(fields).to.match(/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/);

            }

            if (i == 5) {
                expect(fields).not.to.be.NaN;

            }

            if (i == 6) {
                expect(fields).to.be.Nan;

            }

            if (i == 7) {
                expect(fields).to.be.Nan;

            }

            if (i == 8) {
                expect(fields).not.to.be.NaN;

            }

            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

via FuManchu