We calculate the subleading twist contributions to the endpoint of the
inclusive lepton energy spectrum in $B \to X_u \ell \bar\nu$. We show
that the same two subleading twist functions that appear in the decay
$B \to X_s \gamma$ govern the subleading effects in this decay. Using
these results we find large ${O}(\lqcd/m_B)$ corrections to the
determination of $|V_{ub}|$ from the endpoint of the charged lepton
spectrum. Using a simple model for the relevant subleading shape functions,
we estimate the uncertainty in $|V_{ub}|$ from $\lqcd/m_b$ corrections
to be at the $\sim 15\%$ level for a lower lepton energy cut of 2.2 GeV.
