XmlDataSource.Data on PostBack loses it's value

[tMan]

i'm setting the Data property of an XmlDataSource on PageLoad to a xml string

'on page load
XmlDataSource1.Data = "<root>bla bla bla</root>"

but when a PostBack occurs (say on a button click), the Data returns empty
and GetXmlDocument errors with a NullReferenceException

'on a button click event
dim str as string = XmlDataSource1.Data 'this is empty

i tried setting the EnableViewState to tru both on the control and page.

is this XmlDataSource control behavior by design?