getting TypeError: only integer scalar arrays can be converted to a scalar index while using pandas merge code example

Example: only integer scalar arrays can be converted to a scalar index pandas

df1 = pd.DataFrame({'a': [1, 2]})
df2 = pd.DataFrame({'b': [3, 1]})

df1.columns = ['b']

df1.merge(df2, on='b')
#   b
# 0 1