  In a number of papers it has been claimed that the time machine are quantum
unstable, which manifests itself in the divergence of the vacuum expectation
value of the stress-energy tensor $\langle{\bf T}\rangle$ near the Cauchy
horizon. The expression for $\langle{\bf T}\rangle$ was found in these papers
on the basis of some specific approach \cite{Fro,KimT}.\par We show that this
approach is untenable in that the above expression firstly is not derived from
some more fundamental and undeniable premises, as it is claimed, but rather
postulated and secondly contains undefined terms, so that one can neither use
nor check it. As a counterexample we cite a few cases of (two-dimensional)
spacetimes containing time machines with $\langle{\bf T}\rangle$ bounded near
the Cauchy horizon.
