SystemError: error return without exception set

[Shailendra]

Hi All,
I am getting error in "SystemError: error return without exception
set" which i am not able debug. I made following test program which
gives same error.

=========test.py==============

import numpy

class testClass:
    def __init__(self,param1=1,param2=2):
        self.param1,self.param2=param1,param2

    def __str__(self):
        return 'param1=%d param2=%d'%(self.param1,self.param2)


class makeLotOftestClass:
    def __init__(self,paramlist1=None,paramlist2=None):
        self.holder= numpy.empty((len(paramlist1),len(paramlist2)))
        for i in range(len(paramlist1)):
            for j in range(len(paramlist2)):

self.holder[i,j]=testClass(param1=paramlist1[i],param2=paramlist2[j])

    def __str__():
        str=''
        for testclass in numpy.raven(self.holder):
            str=str+testclass.__str__()
        return str

if __name__=='__main__':
    lotofclass=makeLotOftestClass(paramlist1=[10,20,30,40],paramlist2=[110,120,130,140])
    print lotofclass

==============================================

$ python test.py
Traceback (most recent call last):
  File "test.py", line 25, in <module>
    lotofclass=makeLotOftestClass(paramlist1=[10,20,30,40],paramlist2=[110,120,130,140])
  File "test.py", line 16, in __init__
    self.holder[i,j]=testClass(param1=paramlist1[i],param2=paramlist2[j])
SystemError: error return without exception set

Can someone point out why is this exception. It seems to be a simple
code. Google search for the error message did not help much.

Thanks,
Shailendra