  We point out an inconsistency in a method used in the literature for studying
adiabatic scalar perturbations in a regular bouncing universe (in four
dimensions). The method under scrutiny consists of splitting the Bardeen
potential into two pieces with independent evolutions, in order to avoid a
singular behavior at the boundaries of the region where the null energy
condition (NEC) is violated. However, we argue that this method violates
energy-momentum conservation. We then introduce a novel method which provides
two independent solutions for the Bardeen potential around the boundaries, even
in the case of adiabatic perturbations. The two solutions are well behaved and
not divergent.
