  We show that the one-loop effective potential of spontaneously broken N=8
supergravity is calculable and finite at all classical four-dimensional
Minkowski vacua without tachyons in the spectrum. The reason is that the
supertraces of the quadratic and quartic mass matrices vanish along the
classically flat directions: Str M^2 = Str M^4 =0. We also show that Str M^6 =
0 but Str M^8 > 0 in a broad class of vacua with broken supersymmetry on a flat
background, which includes all those explicitly identified so far. We find
analytical and numerical evidence that the corresponding one-loop effective
potential is negative-definite.
