How to set Option set value to null CRM 2011

JavaScript:

function OnLoadForm(){
    setTwoOptionNull("new_tender");
}

function setTwoOptionNull(twoOption) {
    var isCreateForm = Xrm.Page.ui.getFormType() == 1;
    var twoOptionField = Xrm.Page.getAttribute(twoOption);
    var twoOptionValue = twoOptionField.getValue();
    if (isCreateForm) {
        twoOptionField.setValue(null); // set the value to null on create
        twoOptionField.setSubmitMode("always"); // required to store the null value
        document.getElementById("rad_" + twoOption + "1").onclick = function () { 
            if (twoOptionValue == false) {
                twoOptionField.setValue(true);
                twoOptionField.setValue(false);
            }
        }
    }
}