If you are working with Glassfish v1_ur1_b14, PostgreSQL and NetBeans 5.5 probably you had experienced some dificulties with entity class.
(1) you develop the code.
(2) you compile and build your code (an .ear or .war file)
(3) you can't deploy your application :(, Glassfish says you there is an error (with a null desciption) but if you chech your app with the verifier utility it is right.
Well, here are acouple of links describing the problem (related with the @GeneratedValue annotation) and describing a solution: