  The recently discovered 5 MeV bump highlights that the uncertainty in the
reactor neutrino spectrum is far greater than some theoretical estimates.
Medium baseline reactor neutrino experiments will deliver by far the most
precise ever measurements of theta12. However, as a result of the bump, such a
determination of theta12 using the theoretical spectrum would yield a value of
sin^2(2theta12) which is more than 1% higher than the true value. We show that
by using recent measurements of the reactor neutrino spectrum the precision of
a measurement of theta12 at a medium baseline reactor neutrino experiment can
be improved appreciably. We estimate this precision as a function of the 9Li
spallation background veto efficiency and dead time.
