Fluent NHibernate – IndexOutOfRange

I've read all the posts and know that IndexOutOfRange usually happens because a column is being referenced twice. But I don't see how that's happening based on my mappings. With SHOW_SQL true in the config, I see an Insert into the Events table and then an IndexOutOfRangeException that refers to the RadioButtonQuestions table. I can't see the SQL it's trying to use that generates the exception. I tried using AutoMapping and have now switched to full ClassMap for these two classes to try to narrow down the problem.

public class RadioButtonQuestion : Entity
{
    [Required]
    public virtual Event Event { get; protected internal set; }

    [Required]
    public virtual string GroupIntroText { get; set; }
}

public class Event : Entity
{
    [Required]
    public virtual string Title { get; set; }

    [Required]
    public virtual DateTime EventDate { get; set; }

    public virtual IList<RadioButtonQuestions> RadioButtonQuestions { get; protected internal set; }
}




public class RadioButtonQuestionMap : ClassMap<RadioButtonQuestion>
{
    public RadioButtonQuestionMap()
    {
        Table("RadioButtonQuestions");

        Id(x => x.Id).Column("RadioButtonQuestionId").GeneratedBy.Identity();

        Map(x => x.GroupIntroText);
        References(x => x.Event).Not.Nullable();
    }
}


public class EventMap : ClassMap<Event>
{
    public EventMap()
    {
        Id(x => x.Id).Column("EventId").GeneratedBy.Identity();
        Map(x => x.EventDate);
        Map(x => x.Title);
        HasMany(x => x.RadioButtonQuestions).AsList(x => x.Column("ListIndex")).KeyColumn("EventId").Not.Inverse().Cascade.AllDeleteOrphan().Not.KeyNullable();
    }
}

The generated SQL looks correct:

create table Events (
    EventId INT IDENTITY NOT NULL,
   EventDate DATETIME not null,
   Title NVARCHAR(255) not null,
   primary key (EventId)
)

create table RadioButtonQuestions (
    RadioButtonQuestionId INT IDENTITY NOT NULL,
   GroupIntroText NVARCHAR(255) not null,
   EventId INT not null,
   ListIndex INT null,
   primary key (RadioButtonQuestionId)
)

This is using NH 3.3.0.4000 and FNH 1.3.0.727. When I try to save a new Event (with a RadioButtonQuestion attached) I see

NHibernate: INSERT INTO Events (EventDate, Title) VALUES (@p0, @p1);@p0 = 5/21/2012 12:32:11 PM [Type: DateTime (0)], @p1 = 'My Test Event' [Type: String (0)]
NHibernate: select @@IDENTITY

Events.Tests.Events.Tasks.EventTasksTests.CanCreateEvent:
NHibernate.PropertyValueException : Error dehydrating property value for Events.Domain.RadioButtonQuestion._Events.Domain.Event.RadioButtonQuestionsIndexBackref
—-> System.IndexOutOfRangeException : An SqlCeParameter with ParameterIndex '3' is not contained by this SqlCeParameterCollection.

So if a column really is being referenced twice, what's the problem with my FNH config that's causing that behavior? I'm trying for a bidirection relationship (One Event Has Many Radio Button Questions) with ordering (I'll maintain it since NH won't in a bidir relationship, from what I've read). FWIW I also tried this as a unidirectional relationship by removing the Event from RadioButtonQuestion and it still caused the same exception.