Why does an assignment for double-sliced numpy arrays not work?

why do the following lines not work as I expect?

import numpy as np
a = np.array([0,1,2,1,1])
a[a==1][1:] = 3
print a
>>> [0 1 2 1 1]
# I would expect [0 1 2 3 3]

Is this a 'bug' or is there another recommended way to this?

On the other hand, the following works:

a[a==1] = 3
print a
>>> [0 3 2 3 3]

Cheers, Philipp