The entity type is not part of the model for the current context

I am learning Entity Framework, but I'm not sure if I am following the code-first methodology correctly.

I have generated my entities and am using a generic repository pattern based on the code from https://genericunitofworkandrepositories.codeplex.com/.

But I encounter the following issues when I attempt to access or alter the entity:

System.InvalidOperationException: The entity type Estate is not part of the model for the current context.

When I try to access it from my repository, this problem occurs:

public virtual void Insert(TEntity entity)
{
    ((IObjectState)entity).ObjectState = ObjectState.Added;
    _dbSet.Attach(entity); // <-- The error occurs here
    _context.SyncObjectState(entity);
}

The entities (tables) are not created at starting, but the database (./SQLEXPRESS) is built perfectly.

Do I need to explicitly set the mapping of the entities, I wonder. Why can't EF do this on its own?

I am my entity.

public class Estate : EntityBase
{
    public int EstateId { get; set; }
    public string Name { get; set; }
} 

Following is my context:

public partial class DimensionWebDbContext : DbContextBase // DbContextBase inherits DbContext
{
    public DimensionWebDbContext() :
        base("DimensionWebContext")
    {
        Database.SetInitializer<DimensionWebDbContext>(new CreateDatabaseIfNotExists<DimensionWebDbContext>());
        Configuration.ProxyCreationEnabled = false;
    }

    public new IDbSet<T> Set<T>() where T : class
    {
        return base.Set<T>();
    }

}

Is there a particular cause for this error? Without any success, I also tried turning on automatic migrations and enabling migrations.