Javascript 对象在 IE 中为 null 或不是 object

我有一个表单，javascript在其中添加了复选框，当表单提交时，它会检查复选框是否已被选中。这在Firefox或Chrome中工作正常，但在IE7或8中，它会导致错误document.myform.mycheckbox.checked为null或不是对象。

var checkbox = document.createElement("input");
checkbox.type = "checkbox";
checkbox.name = "mycheckbox";
checkbox.value= 291;
var div = document.getElementById("addcb");
div.appendChild(checkbox);
checkbox.checked = false;

在表单标签中，我有onSubmit="return CheckForm（）;"，它在Firefox或Chrome中工作正常，但在IE7或8中，它提交表单而不检查表单或其他表单对象。

if (document.myform.mycheckbox.checked == false){
    errorMsg += "'n'tAgree 't- Please Click I Agree Checkbox";
}
//If there is aproblem with the form then display an error
if ((errorMsg != "") || (errorMsgLong != "")){
    msg = "_______________________________________________________________'n'n";
    msg += "The form has not been submitted because there are problem(s) with the form.'n";
    msg += "Please correct the problem(s) and re-submit the form.'n";
    msg += "_______________________________________________________________'n'n";
    msg += "The following field(s) need to be corrected: -'n";
    errorMsg += alert(msg + errorMsg + "'n" + errorMsgLong);
    return false;
}
return true;

我使用开发人员工具创建了一个报告错误的 Brakpoint：

document.myform.mycheckbox.checked is null or not an object