SyntaxError: unqualified exec is not allowed in ... ?

[j vickroy]

Could someone help me understand the following Python 2.3 error message:

SyntaxError: unqualified exec is not allowed in function 'load' it contains
a nested function with free variables

in the following context:

class Spam(object): pass

class Spammer(object):
    ...
    def load(self):
        ...
        exec 'o = %s()' % Spam.__name__
        ...


Thanks.

P.S.
I would provide a simple script demonstrating the problem, but, so far, I
have not been able to reproduce the error in a simple context.