PreviousPage is null on one machine and not on another.

I have a website with a page that does cross-reference postback using ASP: Button with PostBackUrl as such:

<asp:Button ID="FindButton" runat="server" Text="Find" CssClass="button" PostBackUrl="~/TGS/BusinessResults.aspx"  meta:resourcekey="FindButtonResource1" />

I do not need information about the previous page other than PreviousPage.IsCrossPagePostBack, so adding the type PreviousPageType is not necessary.

Problem I have two cars, one laptop and one desktop. My code works as intended (PreviousPage is not null) on the laptop, but PreviousPage is always null on my desktop with the same code.

I have the same installations of Visual Studio 2010 with the same versions of .NET (used by System.Environment.Version for validation) on both machines.

Any thoughts?

Edit: as an update, this is no longer necessary - I fixed my problem (which actually doesn't have much to do with it) in another question - but, as always; I will include my solution:

I used some Response.Write to write a gif to upload to the website while the actual page was loading, and this messed up some form-related stuff. I included the base target in the header of the downloadable html gif and solved my problem of all links opening in new images.