TypeError: cannot concatenate a non-NDFrame object

I have this DatetimeIndex:

dates = DatetimeIndex(['2017-06-09', '2017-06-10', '2017-06-11', '2017-06-12',
               '2017-06-13', '2017-06-14'],
              dtype='datetime64[ns]', freq='<DateOffset>')

I want to take dates and append them to my DataFrame df:

for i in xrange(0,5):
    df.append(dates[i],ignore_index=True)

I get this error TypeError: cannot concatenate a non-NDFrame object.

UPDATE:

sample data of df:

Out[85]:
2017-06-05   -0.944868
2017-06-06    0.073623
2017-06-07   -0.687232
Freq: <DateOffset>, dtype: float64