insert not creating Id?

I have created a simple object with getters and setters:

public class MemberCanonical : IMemberCanonical
{
    public ObjectId Id { get; set; }
    public String username { get; set; }
    public String email { get; set; }
    public String status { get; set; }
    public IEnumerable<string> roles { get; set; }
}

Now I want to insert a new member into the database with:

try
{
    memberObj.username = username;
    memberObj.email = email;
    memberObj.status = "active";
    // memberObj.Id = new ObjectId().ToString();
    this.membershipcollection.Insert(memberObj, SafeMode.True);
    return true;
}
catch(Exception ex)
{
    return false;   
}

I would expect insert to create a unique _id (Id), but that's not happening. Upon insert when I view the _id field I get "0000000...."

What do I need to do in order for Mongo to generate its own _id on insert?