PostBackURL causes Object reference not set error with Master page

[Fresh_Air_Rider]

Hi

I have two pages which both use the same master page.

Page one contains an asp:textbox control and an asp:button control with
it's PostBackUrl property set to the second page.

The "code behind" of the receiving page contains the following page
which results in "Object reference not set..."

Dim txtTest As TextBox =
CType(Page.PreviousPage.FindControl("txtTest"), TextBox)

Response.Write(txtTest.Text)

I have tried adding "PreviousPageType" and "Reference" directive tags
to the receiving page and all result in the same error message.

This is extremely frustrating and if anyone could suggest what I might
be doing wrong then I'd be very grateful.

I haven't managed to find any "PostBackUrl" examples that use master
pages.

Thanks in anticipation
David