Problem with pickle module python

[cal4]

Hi

I'me tring to convert a program from python 2 to python3 but I've got a problem using python module pickle.
Using python2 I saved a class on a file usign module pickle. When I try to load the class with a python3 script I receive the following message:

AttributeError: ´aaa' object has no attribute 'id'

I believe it's a problem of compatibility between the two version of python because if a save and read the class using the same python versino no problem arise. Can somebody explain me where is the problme and a possible solution.

Thanks a lot

Cal