Exception using `NHibernate.Bytecode.DefaultProxyFactoryFactory`

I am trying to get an MSTest unit test running, and I have the following line in my NHibernate config section in App.config:

<property name="proxyfactory.factory_class">NHibernate.ByteCode.DefaultProxyFactoryFactory, NHibernate</property>

Whenever I try and run the unit test, I get an exception in the ClassInitialize method for the test ficture. The exception is:

NHibernate.Bytecode.UnableToLoadProxyFactoryFactoryException: Unable to load type 'NHibernate.ByteCode.DefaultProxyFactoryFactory, NHibernate.Bytecode' during configuration of proxy factory class.

and my initializer code is:

private static ISessionFactory _sessionFactory;
private static Configuration _configuration;

[ClassInitialize()]
public static void ProductRepositoryInitialize(TestContext testContext)
{
    _configuration = new Configuration();
    _configuration.Configure();
    _configuration.AddAssembly(typeof(Product).Assembly);
    _sessionFactory = _configuration.BuildSessionFactory();
}

Yet the DefaultProxyFactoryFactory is available, as I can instantiate in in a console application using the line:

NHibernate.Bytecode.DefaultProxyFactoryFactory factory = new DefaultProxyFactoryFactory();

So why is NHibernate failing to instantiate, or even find, this type in my unit test class. Is it perhaps the test class initializer is static?