  We evaluate the isospin breaking corrections to the scattering amplitude
\pi^- K^+ -> \pi^- K^+ at threshold in the framework of chiral perturbation
theory. This channel is of particular interest for the strong 2S-2P energy
level shift in pion-kaon bound states. While a prediction of this level shift
is hampered by a large uncertainty in the isoscalar scattering length, we find
only a moderate uncertainty of about 3% in the electromagnetic corrections
which are relevant for the extraction of the scattering lengths from experiment
