getElementById returns null

[samah2100]

My problem is that I have two asp controls textbox and lable of course each one has an id, when i tried to get them using javascript method( getElementById) it returned null

my javascript function is

<script type="text/javascript">
function CharactersChanged()
{
var textbox = document.getElementById("texText");
var counter = document.getElementById("lblCharNum");
if(textbox)
{
if(counter)
{
document.write(counter.value);
var textLength = textbox.value.length;
counter.value = 255 - textLength;
document.write(counter.value);
}
}
}
</script>
why it return null although they are existing (and by the way when i preview the page and view its source code i foud the two controls as expected)