+1  A: 

Did you put a <%@Page%> directive in your Master page? It should only have a <%@Master%> directive.

Kon
Your comment got me thinking, found out the solution. Thanks
Anders
I originally was thinking about asking about the Page directive being on some other included page. But I assumed you weren't including other pages if you're using Master pages. :) Glad you found it in any case.
Kon
A: 

No, the header of my MasterPage is:

<%@ Master Language="VB" CodeFile="theMaster.master.vb" Inherits="theMaster" %>

There is no <%@Page%> directive on the MasterPage.

Anders