CMP in Sun One App7, storing primary key issue

[Mike Horwitz]

I've got a simple one class CMP EJB app. I can create a new record, browse based on the primary key, etc... However, when I try and update a record, I get the following message:
com.sun.jdo.api.persistence.support.JDOUnsupportedOptionException: Update of a primary key field is not allowed.
has anyone come across this?
tia,
Mike