  A recent calculation by Chay and Rey has shown that instantons may make a
significant contribution to the lepton energy spectrum near its endpoint. Using
an ansatz borrowed from the study of high energy baryon number violating
processes, we investigate whether these corrections could spoil the relation
between the nonperturbative contributions to this spectrum and to the photon
energy spectrum in radiative $B$ decays. We find, in general, that this
universality may well fail unless the spectrum is smeared over a region which
is considerably larger than had previously been thought necessary. This result
affects the possibility of performing a reliable measurement of $V_{ub}$ using
inclusive decays.
