numpy.linalg.lstsq gives empty residual

python version: 2.7.6
numpy version: 1.8.0

Here is a short example:

import numpy as np
from numpy.linalg import lstsq

a = np.array([[1, 0], [0, 1]]);
b = np.array([1,2]);

print(lstsq(a,b));

Since a is 2x2 matrix of rank 2, I expect the residual to be 0 instead of empty list according to the doc (http://docs.scipy.org/doc/numpy/reference/generated/numpy.linalg.lstsq.html). But here is what I get:

(array([ 1.,  2.]), array([], dtype=float64), 2, array([ 1.,  1.]))

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