Casting Eval("bitValue") as Bool

I have a list view with a HyperLink control within the ItemTemplate. I want to display the link if a returned value is 0 (false), and not display the link if it is 1 (true).

So far I have this:

<asp:HyperLink runat="server" ID="lnkReview"
  NavigateUrl='<%# Eval("EnquiryID", @"selectcompany.aspx?enq={0}")%>'
  Text="Review Enquiry"
  Visible='<%# ((bool)Eval("Locked"))==true? false : true %>' />

...but this renders an 'Specified cast is not valid' exception.

Examples I've seen elsewhere suugest this should work. I can confirm that the Locked column only returns 0 or 1 (from SQL Server) - surely these should be easily cast from bit/int to bool??