The type 'System.Data.Objects.DataClasses.EntityObject' is defined in an assembly that is not refere

System.Data.Objects.DataClasses.EntityObject with Entity Framework 4

If you for instance create a multi layered application and get the compilation error

The type ‘System.Data.Objects.DataClasses.EntityObject’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089′.

when you compile the business layer just add a reference to System.Data.Entity.