count nans after groupby pandas

1A      B     C
20  foo    one   NaN
31  bar    one  bla2
42  foo    two   NaN
53  bar  three  bla3
64  foo    two   NaN
75  bar    two   NaN
86  foo    one   NaN
9
10df2 = df.C.isnull().groupby([df['A'],df['B']]).sum().astype(int).reset_index(name='count')
11print (df2)
12     A      B  count
130  bar    one      0
141  bar  three      0
152  bar    two      1
163  foo    one      2
174  foo  three      1
185  foo    two      2