I also got this error a moment ago.
I also have a one-to-many relationship.
It happens during redirect after invoking an action. The action code, however seems to load fine.
The indexing operator is for Maps and Lists, not Sets.
Ahh, that makes sense.
I checked the code and the properties are ArrayLists.
As I said I just used seam-gen to generate the code and added an extra relationship in the table .
I then changed the view to additionally display the sub table and pass the key as a request parameter