  $D-\bar D$ mixing is the source of the largest theoretical uncertainty in the
extraction of $\gamma$ from $B\to D K$ decays. In the Standard Model, the
mixing can have a rate close to its current experimental upper bound and is
CP-conserving to an excellent approximation. We show that neglecting
CP-conserving $D-\bar D$ mixing leads to an error in the determination of
$\gamma$ only at second order in the small parameters, $\Delta m_D/\Gamma_D$
and $\Delta\Gamma_D/\Gamma_D$, and is therefore very small and can be safely
neglected.
