  We consider lattice coset-coded transmissions over a wiretap channel with
additive white Gaussian noise (AWGN). Examining a function that can be
interpreted as either the legitimate receiver's error probability or the
eavesdropper's correct decision probability, we rigorously show that, albeit
offering simple bit labeling, orthogonal nested lattices are suboptimal for
coset coding in terms of both the legitimate receiver's and the eavesdropper's
probabilities.
