Entity Framework Code first making a column non-nullable

I am using EF code first for my project. I have following code in my DataModel

[HiddenInput(DisplayValue = false)]        
public DateTime? PasswordDate { get; set; }

To make this non-nullable I removed '?' and ran Add-Migration command from Package manager console. following migration file was generated.

  public partial class PasswordDate : DbMigration
{
    public override void Up()
    {
        AlterColumn("dbo.CertificateInfoes", "PasswordDate", c => c.DateTime(nullable: false));
    }

    public override void Down()
    {
        AlterColumn("dbo.CertificateInfoes", "PasswordDate", c => c.DateTime());
    }
}

But when I run Update-Database command:

Update-Database -SourceMigration 201309020721215_PasswordDate

I get following error: Cannot insert the value NULL into column 'PasswordDate', table ''; column does not allow nulls. UPDATE fails. The statement has been terminated.

Kindly suggest the solutions.