What could be causing this javascript NaN in IE 7?
<p>I have a function that checks if a user writes a digit in an input box:</p>

<pre><code>var letters = 'ABCÇDEFGHIJKLMNÑOPQRSTUVWXYZabcçdefghijklmnñopqrstuvwxyzàáÀÁéèÈÉíìÍÌïÏóòÓÒúùÚÙüÜ';
var numbers = '1234567890';
var signs = ',.:;@-\'';
var mathsigns = '+-=()*/';
var custom = '&lt;&gt;#$%&amp;?¿';


function alpha(event) {
    var k;
    k = document.all ? parseInt(event.keyCode) : parseInt(event.which);
    return (numbers.indexOf(String.fromCharCode(k)) != -1);
}
</code></pre>

<p>This function is used together with the following function, that takes the digit and sends it to a controller, which saves the digit in the database. But only if the digit is different from the original digit, that was in the input box in the first place. It also updates another input box, with the sum off all specific input that have the same row_id attribute as the input box which fired the event:</p>

<pre><code>function initializeUpdateNumberProcentMatrix() {

    $(".numberfield").live('keypress', alpha);

    //$(".numberfield").live('keypress', alpha(numbers));


        $('.numberfield').live('focus', function () {
            $(this).attr('oldvalue', $(this).val());
        });

        $('.numberfield').live('blur', function () {

            // Dette er den nuværende værdi i tekstboksen
            var value = $(this).val();
            var oldvalue = $(this).attr('oldvalue');

            // Reference til tekstboksen
            var obj = $(this);

            // Hvis værdien ikke har ændret sig, skal der ikke foretages noget (hvis man tabulerer)
            if (value == $(this).attr('oldvalue')) {
                return;
            }

            else {

                var dif = value - $(this).attr('oldvalue');
                var newval;


                $('.procentsumfield').each(function (index) {
                    if ($(this).attr('row-id') == obj.attr('row-id')) {

                        newval = (parseInt($(this).val()) + parseInt(dif));

                        // Hvis summen overstige 100, skal der ikke foretages nogle ændringer - Textboksens værdi skal tilbagestilles, og sumfeltet skal ikke odateres                
                        if (newval &gt; 100) {
                            obj.val(oldvalue);
                            alert("Summen for det pågældende år må ikke overstige 100%");
                            return;
                        }
                        else {
                            $(this).val(newval);
                            return false;
                        }
                    }
                });

                var number = { Column_ID: $(this).attr('column-id'),
                    Row_ID: $(this).attr('row-id'),
                    Question_ID: $(this).attr('question-id'),
                    Value: $(this).val(),
                    Type: "Procent"
                };

                // Hvis den nye værdi overstiger 100, skal det ikke gemmes
                if (newval &lt;= 100) {
                    saveNumberMatrix($(this), number, "/SaveSurveyAnswers/SaveProcentMatrix");
                }
            }
        });
    }
</code></pre>

<p>The problem is, that sometimes IE 7 gives me a NaN in the "procentsumfield" input box, and it doesnt save the new value because it's not a number. </p>

<p>What could cause this to happen? </p>

<p>Thanks</p>
