Can't save a numpy 2-d array into a file

I have the following 2-d numpy matrix, which was a concatenation of two matrices:

     >>> mnist1_train_final_data=np.hstack((y_train_mnist_ni,features_train_mnist1))
     >>> type(mnist1_train_final_data)
     <type 'numpy.ndarray'>
     >>> mnist1_train_final_data.dtype
     dtype('S32')
     >>> mnist1_train_final_data.shape
      (1149, 129)

As you can see, it is a 2-d numpy array. However, when I try to save it using the following command:

>>> np.savetxt('test.txt', mnist1_train_final_data, delimiter=',', fmt='%5.2f') 

It shows me the following error:

Traceback (most recent call last):   File "<stdin>", line 1, in
<module>   File "/usr/lib/python2.7/site-packages/numpy/lib/npyio.py",
line 1162, in savetxt
    % (str(X.dtype), format)) TypeError: Mismatch between array dtype ('|S32') and format specifier
('%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f,%5.2f')

How can I save a 2-d numpy matrix in my case?