AutoMapper Issue

i'm using automapper in C# i'm facing an issue in it.

when i get list of objects from database it's fine.

but when i map it to another class then the Id property which is a primery key becomes 0 of all the list objects

*Every thing is fine till here*   

var customers = _context.Customers.ToList(); 

* After this mapping*

var customerList = Mapper.Map, IEnumerable>(customers);  

* This customerList has all the data except the Id property which becomes 0*

Why i'm facing this issue ? how can i fix it ?My both class are below

public class Customer {  
      public int Id { get; set; }      
     public string Name { get; set; }   
     public bool IsSubscribedToNewsLetter { get; set; } 
     public MembershipType MembershipType { get; set; }
}

public class CustomerDto {  
      public int Id { get; set; }      
        public string Name { get; set; }   
}