Mean is incorrectly NaN in 'object' DataFrame

from pandas import DataFrame
df = DataFrame({0: [np.nan, 2], 1: [np.nan, 3], 2: [np.nan, 4]}, dtype=object)
df.mean(1)

Expected:
In [33]: df.mean(1)
Out[33]:
0    NaN
1    3.0

Actual:
In [31]: df.mean(1)
Out[31]:
0    NaN
1    NaN
