Czy ktoś może wskazać, co robię źle tutaj?

import numpy as np

a = np.array([1,2,3,4,5],dtype=int)
b = np.array(['a','b','c','d','e'],dtype='|S1')

np.savetxt('test.txt',zip(a,b),fmt="%i %s")

Dane wyjściowe to:

Traceback (most recent call last):
  File "loadtxt.py", line 6, in <module>
    np.savetxt('test.txt',zip(a,b),fmt="%i %s")
  File "/Users/tom/Library/Python/2.6/site-packages/numpy/lib/io.py", line 785, in savetxt
    fh.write(format % tuple(row) + '\n')
TypeError: %d format: a number is required, not numpy.string_