TypeError: unhashable type: 'list' when use groupby in python

There is something wrong when I use groupby method:

data = pd.Series(np.random.randn(100),index=pd.date_range('01/01/2001',periods=100))
keys = lambda x: [x.year,x.month]
data.groupby(keys).mean()

but it has an error: TypeError: unhashable type: 'list'. I want group by year and month, then calculate the means,why it has wrong?