numpy.fft.rfft producing NaNs -

i'm trying learn bit ffts , bit jupyter/ipython notebook. have few time series data sets. of them, numpy.fft.rfft produces nice set of amplitudes. 1 data set, produces nothing nans. data set looks okay me:

here's 1 happy with:

both have on 40k data points. there way numpy.fft.rfft tell me doesn't data set instead of producing output full of nans?