  Models with non-gravitational interactions between the dark matter and dark
energy components are an alternative to the standard cosmological scenario.
These models are characterized by an interaction term, and a frequently used
parameterization is $Q = 3\xi H \rho_{x}$, where $H$ is the Hubble parameter
and $\rho_{x}$ is the dark energy density. Although current observations
support such a model for negative values of the interaction parameter $\xi$, we
show here that this interval of values of $\xi$ leads the model to predict a
violation of the Weak Energy Condition (WEC) for the dark matter density,
regardless of the value of the equation-of-state parameter of the dark energy
component. This violation is accompanied by unphysical instabilities of matter
perturbations.
