Entity Framework 6 Lazy Loading returns null

I have a class with the following properties

public class Booking
{
    public long BookingId {get;set;}
    public string RoomNumber {get;set;}
    [ForeignKey("BookingCustomer")]
    public long? BookingCustomerId {get;set;}

    public virtual Customer BookingCustomer {get;set;}
}

public class Customer
{
    public long CustomerId {get;set;}
    public string FirstName {get;set;}
}

if in a method I reference properties of the customer class am getting object null reference exception while BookingCustomerId is populated.i.e.,

 hotel.BookingCustomerId=2

For instance, string customerFirstName = hotel.BookingCustomer.FirstName; if I peek at the hotel.BookingCustomer i get null How do I go about this Lazy Loading?