为什么复制XML节点时会丢失命名空间引用?

<my:ClientMaintenance xmlns:my="http://schemas.microsoft.com/office/infopath/2003/myXSD/2009-07-06T07:04:54">
    <my:Payments>
    </my:Payments>
    <my:Payment>
        <my:Amount></my:Amount>
        <!-- Several other nodes -->
    </my:Payment>
</my:ClientMaintenance>

这个 Payment Payments 节点。所以每当 添加付款 按钮,我需要:

1. 内存中的节点
3. 将其作为子级附加到 付款 节点

my 命名空间!代码如下:

public void btnAddPaymentClicked(object sender, ClickedEventArgs e)
{
    var navigator = e.Source.CreateNavigator();
    var blankPayment = GetBlankPaymentNode(navigator).CreateNavigator();
    // new XmlNamespaceManager(blankPayment.NameTable).HasNamespace("my") == false
    // WHY???
}

private XmlDocument GetBlankPaymentNode(XPathNavigator navigator)
{
    var blankPayment = navigator.SelectSingleNode(FullBlankPaymentXPath, NamespaceManager);
    var blankXml = new XmlDocument();
    blankXml.LoadXml(blankPayment.OuterXml);
    var schema = new XmlSchema();
    schema.Namespaces.Add("my", MyNamespaceUri);  // Set elsewhere and != null
    blankXml.Schemas.Add(schema);
    return blankXml;
}

为什么命名空间不能再被引用?