[Python] Pandas Merge duplicate index into single index

I have a simple data frame like this:

d1={'a':{'1998-01-01':10}}
d2={'b':{'1998-01-01':3}}

df=pd.DataFrame.from_dict(d1)
df=df.append(pd.DataFrame.from_dict(d2))
df.index=pd.to_datetime(df.index)

a b
1998-01-01 10 NaN
1998-01-01 NaN 3


I would like to have

a b
1998-01-01 10 3


Since 1998-01-01 share the index

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