Fluent NHibernate one-to-many relationship sets foreign key to null

I have a simple Fluent NHibernate model with two related classes:

public class Applicant
    {
        public Applicant()
        {
            Tags = new List<Tag>();
        }

        public virtual int Id { get; set; }

        //other fields removed for sake of example

        public virtual IList<Tag> Tags { get; protected set; }

        public virtual void AddTag(Tag tag)
        {
            tag.Applicant = this;
            Tags.Add(tag);
        }
    }


public class Tag
{
    public virtual int Id { get; protected set; }
    public virtual string TagName { get; set; }

    public virtual Applicant Applicant { get; set; }
}

My free mapping is as follows:

public class ApplicantMap : ClassMap<Applicant>
    {
        public ApplicantMap()
        {
            Id(x => x.Id);

            HasMany(x => x.Tags).Cascade.All();
        }
    }

    public class TagMap : ClassMap<Tag>
    {
        public TagMap()
        {
            Id(x => x.Id);
            Map(x => x.TagName);

            References(x => x.Applicant).Not.Nullable();
        }
    }

Whenever I try to update a candidate (adding a new one works fine), it fails and I see the following SQL exception in the logs:

11:50:52.695 [6] DEBUG NHibernate.SQL - UPDATE [Tag] SET Applicant_id = null WHERE Applicant_id = @p0;@p0 = 37 [Type: Int32 (0)] 
11:50:52.699 [6] ERROR NHibernate.AdoNet.AbstractBatcher - Could not execute command: UPDATE [Tag] SET Applicant_id = null WHERE Applicant_id = @p0 System.Data.SqlClient.SqlException (0x80131904): Cannot insert the value NULL into column 'Applicant_id', table 'RecruitmentApp.dbo.Tag'; column does not allow nulls. UPDATE fails.

Why is NHibernate trying to update the tag table and set Applicant_id to null? I'm at a loss on this.