Obscure NHibernate/Fluent NHibernate error

I've been encountering the following error when trying to build a
session factory:

PersistenceTests.Can_Map_Orders_To_Database : Failed 
System.IndexOutOfRangeException: Index was outside the bounds of the 
array. 
at NHibernate.Mapping.Column.set_Name(String value) 
at NHibernate.Cfg.XmlHbmBinding.ClassBinder.BindColumns(XmlNode node, 
SimpleValue model, Boolean isNullable, Boolean autoColumn, String 
propertyPath) 
at NHibernate.Cfg.XmlHbmBinding.ClassBinder.BindColumnsOrFormula 
(XmlNode node, SimpleValue simpleValue, String path, Boolean 
isNullable) 
at NHibernate.Cfg.XmlHbmBinding.ClassBinder.BindSimpleValue(XmlNode 
node, SimpleValue model, Boolean isNullable, String path) 
at 
NHibernate.Cfg.XmlHbmBinding.CollectionBinder.BindCollectionSecondPass 
(XmlNode node, Collection model, IDictionary`2 persistentClasses) 
at 
NHibernate.Cfg.XmlHbmBinding.CollectionBinder.<>c__DisplayClassd.<AddCollec tionSecondPass>b__c 
(IDictionary`2 persistentClasses) 
at NHibernate.Cfg.Configuration.SecondPassCompile() 
at NHibernate.Cfg.Configuration.BuildSessionFactory() 
at FluentNHibernate.Cfg.FluentConfiguration.BuildSessionFactory() in c: 
\Code Samples\NHibernate\Fluent Nhibernate - Trunk\src\FluentNHibernate 
\Cfg\FluentConfiguration.cs: line 94 
FluentNHibernate.Cfg.FluentConfigurationException: An invalid or 
incomplete configuration was used while creating a SessionFactory. 
Check PotentialReasons collection, and InnerException for more detail. 
at FluentNHibernate.Cfg.FluentConfiguration.BuildSessionFactory() in c: 
\Code Samples\NHibernate\Fluent Nhibernate - Trunk\src\FluentNHibernate 
\Cfg\FluentConfiguration.cs: line 99 
at FluentNHibernate.SessionSource..ctor(FluentConfiguration config) in 
c:\Code Samples\NHibernate\Fluent Nhibernate - Trunk\src 
\FluentNHibernate\SessionSource.cs: line 38 
at 
FluentNHibernate.Testing.SingleConnectionSessionSourceForSQLiteInMemoryTest ing..ctor 
(FluentConfiguration config) in c:\Code Samples\NHibernate\Fluent 
Nhibernate - Trunk\src\FluentNHibernate\Testing 
\SingleConnectionSessionSourceForSQLiteInMemoryTesting.cs: line 15 
at Core.Infrastructure.Data.NHibernate.Tests.PersistenceTests.SetUp() 
in PersistenceTests.cs: line 26

I'm working against the FNH trunk and NH 2.0.1. Funny thing is I am
able to compile my mappings (via AutoPersistenceModel.CompileMappings)
and write them to the file system successfully – FNH doesn't complain.
It is only when attempting to build the session factory that
everything goes kaboom with the not very helpful error message above.
Anyone got any ideas?