python 2.7 - Trouble with pandas merge function -

i trying merge 2 panda's data frame using merge function. here top few lines of both data frames.

newdf     0   1x 0   1   farm products 1   11  field crops 2   112 cotton, raw 3   1129    raw cotton, nec 4   1131    barley 

and

df_2012     0   1          2           3    4    0   1   657042  52539820    103009  10818958     1   11  613499  51051986    99650   10525341     2   112 10710   233790         0    0     3   1121    0   0              0    0           4   1131 13276  998109       3560   0 

when merge following command:

pd.merge(newdf, df_2012, on = [newdf.columns[0]], how = 'right') 

i getting following:

    0   1x  1   2   3            4 0   1   nan 657042  52539820    103009   1   11  nan 613499  51051986    99650    2   112 nan 10710   233790  0   0    3   1121    nan 0   0   0   0   0    4   1131    nan 13276   998109  3560     

i don't understand why new merged data frame's '1x' column has nans. thought keys doesn't appear on left data frame, in newdf, appear nans. rows keys matches in both data frames show other column values both data frames.

what missing here ?

thanks input make me understand problem.