  It is shown that the inclusive rate of the rare weak radiative decays $B \to
X_s \gamma$ contains a series of nonperturbative corrections, whose `short
distance' scale is set by $m_c^{-1}$, rather than by $m_b^{- 1}$. The first
correction in this series is expressed through the chromomagnetic interaction
of the $b$ quark inside the $B$ meson and the relative magnitude of the effect
is determined by the ratio $< {\overline B} | \bar b \sigma \cdot G b | B > /
m_c^2$. Though the magnitude of this first correction is suppressed by a
numerical coefficient, the sensitivity of the decay rate to the distance scale
$m_c^{-1}$ may significantly limit the accuracy of purely perturbative
predictions for the rate.
