Given a finite set of roots of unity, we show that all power sums are nonnegative integers iff the set forms a group under multiplication. The main argument is purely combinatorial and states that for an arbitrary finite set system the non-negativity of certain alternating sums is equivalent to the set system being a filter. As an application we determine all discrete Fourier pairs of {0, 1}-matrices. This technical result is an essential step in the classification of R-matrices of quantum groups.