  The maximum operational range of continuous variable quantum key distribution
protocols has shown to be improved by employing high-efficiency forward error
correction codes. Typically, the secret key rate model for such protocols is
modified to account for the non-zero word error rate of such codes. In this
paper, we demonstrate that this model is incorrect: Firstly, we show by example
that fixed-rate error correction codes, as currently defined, can exhibit
efficiencies greater than unity. Secondly, we show that using this secret key
model combined with greater than unity efficiency codes, implies that it is
possible to achieve a positive secret key over an entanglement breaking channel
- an impossible scenario. We then consider the secret key model from a
post-selection perspective, and examine the implications for key rate if we
constrain the forward error correction codes to operate at low word error
rates.
