ASP.Net Profiles - Value cannot be null.Parameter name: type

This is the second time I've come across the error "Value cannot be null.Parameter name: type" when using ASP.Net Membership Profiles.

Profiles are great, they allow you to store little pieces of information e.g. their user id (an integer reference to your database) on the user against their User object. You can then use that as a property of the User which can get you out of a bind or two.

Since switching to Web Deployment projects to get around a few issues with multiple environment configuration switching however I started to get "Value cannot be null.Parameter name: type". After a little Googling around I found that it relates to the setting "Treat as library component (remove the App_Code.compiled file)" setting under the property pages.

Un-checking the box sorts all your woes :)