XML Serialization + Namespace (C #)

I need to generate something like this:

<AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">
<Header>
...
</Header>
</AmazonEnvelope>

I tried something like this, but this is not entirely correct:

XmlSerializerNamespaces nms = new XmlSerializerNamespaces();
        nms.Add("xsi", "http://www.w3.org/2001/XMLSchema-instance");
        nms.Add("noNamespaceSchemaLocation", "amzn-envelope.xsd");



        XmlSerializer serializer = new XmlSerializer(typeof(XMLAmazonEnvelope));
        StreamWriter writer = new StreamWriter(path);

        serializer.Serialize(writer, objectToSave,nms);
        writer.Close();

And the result:

<?xml version="1.0" encoding="utf-8"?>
<xsi:AmazonEnvelope xmlns:noNamespaceSchemaLocation="amzn-envelope.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<xsi:Header>
...
</xsi:Header>
</xsi:AmazonEnvelope>

And that’s not quite what I want. Any ideas what needs to be done differently?