JaavaScript Form Validation not working using isNaN

I have changed my code and added the isNaN to make sure a credit card validation is checking that only numbers are entered, however it broke the code.

Note this is only a code snippet of the broken code. Tried adding backets and moving around code.

    var american = document.getElementById("americanInput").value;
    var master = document.getElementById("masterInput").value;
    var visa = document.getElementById("visaInput").value;
    var email = document.getElementById("email").value;
    var errMsg = document.getElementById("errMsg");
    var postcode = document.getElementById("billingPostcode").value;
    var address = document.getElementById("billingAddress").value;
    var suburb = document.getElementById("billingSuburb").value;
    } else if (master.length > 0 && master.length !== 16 && isNaN(master)||
        visa.length > 0 && visa.length !== 16 && isNaN(visa)||
        american.length > 0 && american.length !== 15 && isNaN(american)){
        errMsg.innerHTML = "Please check your card details again";
        return false;
    } else if (postcode.length < 4 || (!postcode.match(/^[0-9]*$/))) {
        errMsg.innerHTML = "Please enter a valid postcode";
        return false;
    } else if (suburb.length < 8 || address.length < 8) {
        errMsg.innerHTML = "Please check your address, to confirm details"
        return false;
    }

The code should validate and give an error message.