XmlDocument and VB.NET

I have an XmlDocument with a root node

<root>

</root>

I want to add the following attributes to the root node:

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
and
xsi:noNamespaceSchemaLocation="../XSD/schema1.xsd"

so that my document look like this:

<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../XSD/schema1.xsd".

</root>

here is my code to accomplish that:
''add attributes to the root node
root.SetAttribute("xmlns:xsi", "http://www.w3.org/2001/XMLSchema-instance")
root.SetAttribute("xsi:noNamespaceSchemaLocation", "../XSD/schema1.xsd")

It works but for some reason in the second attribute it omits the "xsi:" prefix.

So instead of

<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../XSD/schema1.xsd".

</root>


I get

<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" noNamespaceSchemaLocation="../XSD/schema1.xsd".

</root>

Why is it happening and how can I fix it?