Difficulty in implementing Strassen's algorithm in Python

I don't understand how to call my code recursively. Here is my code so far:

import numpy

B = [[5,5,5,5,5,5,5,5],[6,6,6,6,6,6,6,6],[7,7,7,7,7,7,7,7],[8,8,8,8,8,8,8,8],
 [9,9,9,9,9,9,9,9], [10,10,10,10,10,10,10,10],[11,11,11,11,11,11,11,11],       [12,12,12,12,12,12,12,12]]

A = [[5,5,5,5,5,5,5,5],[6,6,6,6,6,6,6,6],[7,7,7,7,7,7,7,7],[8,8,8,8,8,8,8,8],
 [1,1,1,1,1,1,1,1], [2,2,2,2,2,2,2,2],[3,3,3,3,3,3,3,3],[4,4,4,4,4,4,4,4]]

def main():
   strassen(A,B)

def strassen(A, B):
    A = numpy.asarray(A)
    B = numpy.asarray(B)
    lengthA = len(A)
    lengthB = len(B)
    if lengthA == 2:
        print "will calculate"
    else:       
        a, b = strassen(A[:lengthA//2, :lengthA//2], B[:lengthB//2, :lengthB//2])

        lengthA = lengthA//2
        lengthB = lengthB//2
        print a
        print b
        return a, b

I'm trying to reduce a to [[5,5],[6,6]] and b to [[5,5],[6,6]] but I'm getting an error:

a, b = strassen(A[:lengthA//2, :lengthA//2], B[:lengthB//2, :lengthB//2])
TypeError: 'NoneType' object is not iterable. 

a and b are the 1st 2x2 matrices that will be formed after the 2nd whole matrix division for A and B. Please can someone explain this to me. Thanks