  Liu et al. [Phys. Rev. E 74, 056302 (2006)] have presented a WKB analysis of
the helical magnetorotational instability (HMRI), and claim that it does not
exist for Keplerian rotation profiles. We show that if radial boundary
conditions are included, the HMRI can exist even for rotation profiles as flat
as Keplerian, provided only that at least one of the boundaries is sufficiently
conducting.
