The DC bin of an FFT should be the mean of all the other bins in the spectrum, why does the FFT produce 0?
Depending on your normalization factor the DC bin is either the sum of the signal values in your frame (1), the average (1/N), a "representation" (1/sqr(N)),

In the DFT formula the exponential value is $e^0$, or 1, for each term.