NHibernate Fluent Mapping results in Invalid index n for this SqlParameterCollection with Count=n - nhibernate

Question

I have a class TrainingSpecialty which is mapped as follows:
public TrainingSpecialityMap()
{
Table("TrainingSpecialty");
Id(o => o.Id).GeneratedBy.Native();
DiscriminateSubClassesOnColumn(CandidateTrainingDatabase.TrainingSpecialtyTable.IsCombinedColumn)
.Formula(
string.Format("case when {0} is null then 0 else {0} end", CandidateTrainingDatabase.TrainingSpecialtyTable.IsCombinedColumn));
Map(o => o.Code);
Map(o => o.Name).Not.Nullable();
Map(o => o.TrainingProgramType, "ProgramType").Not.Nullable();
HasMany(o => o.Forms)
.Not.KeyNullable()
.KeyColumn("TrainingSpecialtyID").Cascade
.SaveUpdate();
Map(o => o.ProcedureTitle).Nullable();
HasMany(o => o.Procedures);
}
I'm attempting to create a new Form, add it to the TrainingSpecialty.Forms list and then update the TrainingSpecialty. The purpose of this is to add the new form. (Note that Form does NOT have a navigation property back to the TrainingSpecialty. Without changing the Form class, there is no simple way to just add the form with a reference to the TrainingSpecialty, which is why I'm updating the TrainingSpecialty.)
The first error I'm getting upon Updating the TrainingSpecialty is Error dehydrating property value for Domain.EvaluationForm.Domain.TrainingSpecialty.FormsBackref.
The inner exception is Invalid index 3 for this SqlParameterCollection with Count=3.
There should always be one previously existing form and I'm now attempting to add a second. I've looked for fields that are duplicately mapped (as per others who have had this issue).
How is this mapping off?

Answer 1

It was a duplicate mapping, just like so many others like this. In this case the Form mapping someone had added a reference back to the Training Specialty.