JavaScript: document.getElementById(txtlatternumer).value returns NULL value

i have write the asp.net  page then error is not showing code is working  but i had write the code in content page the error is showing. and not pick the javascript file ..(bcz i am adding the java script file in content page so help me..error in line  tVal = document.getElementById('txtlatternumer').value; it return the null value..

<script language="javascript" type="text/javascript">

    function MemberNo(mValue) {
        if (event.keyCode > 47 && event.keyCode < 58) {
            var tVal = 0;
            var keycharacter = 0;
            tVal = document.getElementById('txtlatternumer').value;
            keycharacter = String.fromCharCode(event.keyCode);
            tVal = tVal + keycharacter;
            if (parseInt(tVal) > mValue) {
                return false;
            }
        } else { return false; }
    }
</script>