Using Moq and Unit testing with EF6

I have started to use Moq and so far, i think i have worked out how to abstract my interface to another layer to support Mocking the DbContext.

Now my code is running and i hit this line

_context.Entry(updated).Property(e => e.Position).IsModified = false;

Now the entry is null, so i get an error. How can i make Moq happy?

Here is most of my IDataContext

public interface IDataContext
{
    IDbSet<Content> Contents { get; set; }
    IDbSet<FieldGroup> FieldGroups { get; set; }

    #region DbContext public properties

    DbChangeTracker ChangeTracker { get; }
    DbContextConfiguration Configuration { get; }
    Database Database { get; }

    #endregion

    #region DbContext public methods

    IEnumerable<DbEntityValidationResult> GetValidationErrors();
    DbEntityEntry Entry(object entity);
    DbEntityEntry<TEntity> Entry<TEntity>(TEntity entity) where TEntity : class;
    DbSet<TEntity> Set<TEntity>() where TEntity : class;
    DbSet Set(Type entityType);
    int SaveChanges();
    Task<int> SaveChangesAsync();

    #endregion

    #region State

    bool IsState(object entity, EntityState state);
    void SetState(object entity, EntityState state);

    #endregion
}

To cope with setting state on an object, i then abstracted the following to methods to my DataContext

public bool IsState(object entity, EntityState state)
{
    return this.Entry(entity).State == state;
}


public void SetState(object entity, EntityState state)
{
    this.Entry(entity).State = state;
}

But i am not sure how to cope with the Property problem.

Any help much appreciated