  We study nonperturbative corrections up to O(1/m_b^3) in the inclusive rare B
decay B -> X_s l^+ l^- by performing an operator product expansion. The values
of the matrix elements entering at this order are unknown and introduce
uncertainties into physical quantities. Imposing a phase space cut to eliminate
the $c \bar{c}$ resonances we find that the O(1/m_b^3) corrections introduce an
O(10%) uncertainty in the measured rate. We also find that the contributions
arising at O(1/m_b^3) are comparable to the ones arising at O(1/m_b^2) over the
entire region of phase space.
