JavaScript runtime error: Unable to get property rows of undefined or null reference

i get error when i try to get presse arrow key down please help

<script type="text/javascript">
    function enter(obj) {
        var tr = obj.parentNode.parentNode;
        var cellIndex = obj.parentNode.cellIndex;
        var rowIndex = obj.parentNode.parentNode.rowIndex;
        if (event.keyCode == 37) //Left
        {
            if (cellIndex > 0)
                tr.parentNode.rows[rowIndex].cells[cellIndex - 1].getElementsByTagName('INPUT')[0].focus();
            return;
        }
        if (event.keyCode == 38) //Up
        {
            if (rowIndex > 1)
                tr.parentNode.rows[rowIndex - 1].cells[cellIndex].getElementsByTagName('INPUT')[0].focus();
            return;
        }
        if (event.keyCode == 39) //Right
        {
            if (cellIndex < tr.cells.length - 1)
                tr.parentNode.rows[rowIndex].cells[cellIndex + 1].getElementsByTagName('INPUT')[0].focus();
            return;
        }
        if (event.keyCode == 40) //Down
        {
            if (rowIndex < tr.parentNode.rows.length - 1)
                tr.parentNode.rows[rowIndex + 1].cells[cellIndex].getElementsByTagName('INPUT')[0].focus();
            return;
        }
    };
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm != null) {
        prm.add_endRequest(function (sender, e) {
            if (sender._postBackSettings.panelsToUpdate != null) {
               // enter(obj);
            var rowIndex = document.getElementById("<%=hfRowIndex.ClientID%>").value;
                alert(rowIndex);

                ///var row = document.getElementById("ContentPlaceHolder1_gvCustomers").rows.rowIndex + 1;
                var row = document.getElementById("gvCustomers").rows[rowIndex + 2];


                alert(row);
                if (typeof (row) != "undefined") {
                    var celIndex = parseInt(document.getElementById("<%=hfCellIndex.ClientID%>").value);
                    var controls = row.cells[celIndex].getElementsByTagName("INPUT");
                    for (var i = 0; i < controls.length; i++) {
                        if (controls[i].id.indexOf("txtCountry") || controls[i].id.indexOf("txtName") != -1) {
                            // Focus to the next record.
                            controls[i].focus();
                            break;
                        }
                    }
                }
            }
        });
    };
</script>

error is flagged when i reach here

var row = document.getElementById("gvCustomers").rows[rowIndex + 2];