Outer Join Two Pandas Dataframes

Question

I have three pandas data frames that look like:

import pandas as pd
df1=pd.dataFrame ({
                 'Bus':['AM','AM','WM','PC','CC'],\
                 'IC/Third':['IC','Third','Third','IC','Third'],\
                 'Line':['PRV','PRV1','PRV2','PRV3','PRV4'],\
                 'Amount':[100,100,100,100,100]\
                  })
df1=df1.groupby(['Bus','IC/Third','Line']).agg({'Amount':'sum'})
df1.reset_index()

df2=df1.groupby(['Bus','Line']).agg({'Amount':'sum'})
df2.reset_index()

df3=pd.merge(df1,df2,on=['Bus','Line'],how='left')
df3

I am trying to join dataframes df1 and df2 but get an error. Any ideas why ?