about sort and dictionary

[Shi Mu]

Got confused by the following code:
>>> a
[6, 3, 1]
>>> b
[4, 3, 1]
>>> c
{1: [[6, 3, 1], [4, 3, 1]], 2: [[6, 3, 1]]}
>>> c[2].append(b.sort())
>>> c
{1: [[6, 3, 1], [1, 3, 4]], 2: [[6, 3, 1], None]}
#why c can not append the sorted b??
>>> b.sort()
>>> b
[1, 3, 4]