Generation of designer file failed: Unknown server tag 'W3b:Form'

I've got a class which inherits System.Web.UI.Form (and a few other control classes) in a separate assembly.

When my Web Application references the assembly (and I add the reference to Web.config under ) <i>most</i> of the controls in the assembly show up in the HTML/ASP.NET Intellisense window. But some of the controls (including my custom form control) don't.

Furthermore, when I type them in by hand the IDE doesn't recognise them and spits out:

Warning*** 1*** Generation of designer file failed: Unknown server tag 'W3b:Form'.*** Z:\proj\Foo\Default.aspx*** 16*** 0*** W3b.Foo.Web

I've rebuilt everything several times, ensured all the attributes are fine, and restarted the IDE even more times, all to no help.

Any ideas?