unpickleable why?

shell.py

import wat
from pickle import dumps
try:
    wat.b()
except Exception, the_exc:
    exc = the_exc

print dumps(exc)

wat.py

class FrobExc(Exception):
    pass

class OtherExc(Exception):
    pass

def a():
    raise FrobExc('wark`')

def b():
    try:
        a()
    except FrobExc, e:
        exc_msg = str(e)
        # munge exc but keep original traceback.
        # gross, but see: http://blog.ianbicking.org/2007/09/12/re-raising-exceptions/
        import sys
        exc_class, exc, tb = sys.exc_info()
        new_exc = OtherExc()
        raise new_exc.__class__, new_exc, tb