Composite keys on a mapping table runtime error

I have 3 tables:
Entity with EntityID as key
EntityType with EntityType as key
EntityTypeMappings This one is the composite key of EntityID and EntityType

So one Entity can be of more than one type.

My map class for Fluent is as follows:

     public EntityToEntityTypeLinkMap() {
        Table("EntityToEntityTypeLink");
        LazyLoad();       

        CompositeId().KeyReference(x => x.EntityID).KeyReference(x => x.EntityTypeID);
        //References<Entity>(x => x.Entity).Column("EntityID");
       //References<EntityType>(x => x.EntityType).Column("EntityTypeID");
        Map(x => x.CreatedDate).Column("CreatedDate");
        Map(x => x.CreatedBy).Column("CreatedBy");
        Map(x => x.UpdatedDate).Column("UpdatedDate");
        Map(x => x.UpdatedBy).Column("UpdatedBy");
        Map(x => x.Version).Column("Version");
    }

When this is run, I get an error that the class can not compile. I have the XML output which is as follows:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="property" auto-import="true" default-cascade="none" default-lazy="true">
    <class xmlns="urn:nhibernate-mapping-2.2" lazy="true" mutable="true" name="CFIP.Converts.NHibernate.EntityToEntityTypeLink, CFIP.Converts.NHibernate, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="EntityToEntityTypeLink">
        <composite-id mapped="false" unsaved-value="undefined">
            <key-many-to-one name="EntityID" class="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
                <column name="EntityID" /> 
            </key-many-to-one>
            <key-many-to-one name="EntityTypeID" class="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
                <column name="EntityTypeID" /> 
            </key-many-to-one>
        </composite-id>
        <property name="CreatedDate" type="System.Nullable`1[[System.DateTime, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
            <column name="CreatedDate" /> 
        </property>
        <property name="CreatedBy" type="System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
            <column name="CreatedBy" /> 
        </property>
        <property name="UpdatedDate" type="System.Nullable`1[[System.DateTime, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
            <column name="UpdatedDate" /> 
        </property>
        <property name="UpdatedBy" type="System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
            <column name="UpdatedBy" /> 
        </property>
        <property name="Version" type="System.Nullable`1[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
            <column name="Version" /> 
        </property>
    </class>
</hibernate-mapping>

If I comment out the composite key section and put in a standard key statement, everything runs fine. Any ideas/suggestions would be greatly appreciated. Thanks.