Parsing NHibernate Exception Text

I am trying to get NHibernate to load some records for me (it was partially configured and already used for some other parts of the application) and while working on the <any> mapping I got this exception:

[InvalidOperationException: any types do not have a unique referenced persister]

      

        
        
        
      

    

Can someone help me figure out what they mean by this? I can think of many very different meanings for this sentence. I can interpret the first part as:

• types declared with <any> don't have a URP, but yours
• types declared with <any> must have a URP, but you don't
• any of your program types should ...

And with any of these, I see the second part as:

• you have more than one user, but only one is allowed.
• you don't have any permanent but it takes one
• you have one but failed to edit it correctly

(Yes, I still don't know much of my terminology, but usually when I'm unclear about some parts, the error messages are at least clear enough for me to understand what they mean by context. To the entry point in NHibernate rather than bad mapping in my .hbm.xml file or in a property in a specific class.)

I have looked at the API docs but they seem completely useless.

thanks!