This problem was reported by Frank Schwarz on the user forum. It appeared to be related to the use of the new element-collection element in XML, but is actually are result of not having all persistent fields defined in XML. If all fields are defined - or the entity is simply annotated with @Entity no exception is thrown.
Based on the JPA spec:
2.1.1 - If the entity has field-based access, the persistence provider runtime accesses instance variables
directly. All non-transient instance variables that are not annotated with the Transient
annotation are persistent. When field-based access is used, the object/relational mapping annotations
for the entity class annotate the instance variables.
In addition, the <entity> xsd documentation states: [entity] Defines the settings and mappings for an entity. Is allowed to be
sparsely populated and used in conjunction with the annotations.