  A theoretical analysis has been performed of the HERMES semi-inclusive deep
inelastic data on the difference between $\pi^{+}$ and $\pi^{-}$
multiplicities. It turns out that the application of a standard perturbative
QCD analysis, using, as usual, isospin symmetry for the fragmentation
functions, leads to a poor description of the deuteron data at small z values.
If one allows a breaking of isospin invariance, a good fit to both proton and
deuteron data can be achieved for all measured z, but the level of isospin
violation, especially at small values of z, is simply not credible. We suspect
that the problem is a consequence of using the factorized QCD treatment in a
kinematic region where it is unjustified.
