pandas dataframe substracting columns: key error

[metalray]

Hi Pandas Experts,

I got a dataframe that has a country and its GDP for the year 2015 up to 2006

    newfour = newtwo[(newtwo['2015']-newtwo['2006'])]

When I try to substract 2015 from 2006 and ingone the rest I read the error:

KeyError: '[  4.80566748e+12   1.75626949e+12   1.73021626e+11   2.46702696e+11\n   2.30356251e+11   2.28139911e+11   3.52665152e+11   1.10131180e+12\n   1.53345695e+11   3.25560528e+11  -1.52854060e+11   4.99867550e+09\n              nan   2.79311766e+11   4.74343726e+11] not in index'

It seems the GDP values are not an/in index but I wonder why that is required?
Why does a column need to be an index to be subtracted from another?
Many thanks for any explanations