Extending MembershipUser

I'm writing a custom MembershipProvider which uses a custom class derived from MembershipUser (basically, the derived class adds a
field to the MembershipUser base class).

When I try to configure my website using the ASP.NET Configuration tool, and go to the Security page, I get the following error:

Could not load type 'OMLMembershipUser' from assembly 'App_Code.olsckwwk, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

Yet OMLMembershipUser is defined in a file in the App_Code directory.

Any idea what's causing this problem?

- Mark
.