  Extremal six-dimensional black string solutions with some non-trivial
momentum distribution along the wave are considered. These solutions were
recently shown to contain a singularity at the would-be position of the event
horizon. In the black string geometry, all curvature invariants are finite at
the horizon. It is shown that if the effects of infalling matter are included,
there are curvature invariants which diverge there. This implies that quantum
corrections will be important at the would-be horizon. The effect of this
singularity on test strings is also considered, and it is shown that it leads
to a divergent excitation of the string. The quantum corrections will therefore
be important for test objects.
