Entity Framework 6, cannot use Ignore method on the property

I'm currently utilizing MVC 5 and ASP Identity. The AspNetUsers table has a phone number field that I wish to remove, but using the add-migration command results in the following error.

You cannot use Ignore method on the property 'PhoneNumber' on type 'Models.DbModel.User' because this type inherits from the type 'Microsoft.AspNet.Identity.EntityFramework.IdentityUser`

I've previously read a ton of questions on this site, but every single one of them indicated that you should ignore properties in your base class; however, in my situation, I don't have access to the base.

How can I fix this issue?

Update: Since I didn't want to utilize fluent API in this manner, I created distinct config classes for each object. When using fluent API inside the OnModelCreating method, everything worked.

Here's my code:

Descending Entity Class

public class User: IdentityUser
{
    public  ICollection<Comment> Comments { get; set; }

}  

Setting class

public sealed class UserConfig : EntityTypeConfiguration<User>
{
    public UserConfig()
    {
        ToTable("dbo", "Users");
        Ignore(x => x.PhoneNumber);
        Ignore(x => x.PhoneNumberConfirmed);
    }
}  

Context Type

public class WebsiteContext : IdentityDbContext
{
    public WebsiteContext()
        : base("XYZ")
    {
    }

    public DbSet<Comment> Comment { get; set; }
    //public DbSet<User> User { get; set; }

    public static WebsiteContext Create()
    {
        return new WebsiteContext();
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Configurations.Add(new CommentConfig());
        modelBuilder.Configurations.Add(new UserConfig());
    }
}