IE错误：无法设置属性innerHTML的值

Question

function MapIt(actionUrl, slistngs) {

    var mlNums = $form.find("#Mlnums").val();
    var sHTML = "<form id='form1' action='" + actionUrl + "' method='post' target='MLmap'>";

    sHTML += "<input name= mlnum type='hidden' value='" + mlNums + "' />";
    sHTML += "</form>";

    var frmTosubmt = window.open(actionUrl, "MLmap", "width=800,height=550, menubar=0, resizable=0,scrollbars=0");
    frmTosubmt.document.body.innerHTML = sHTML;
    frmTosubmt.form1.submit();
}

使用上面的代码，我在第unable to set property 'innerhtml' of undefined or null reference行收到以下错误frmTosubmt.document.body.innerHTML = sHTML;，这只发生在IE中。 chrome和firefox工作得很好。