  Using the virial theorem of the heavy-quark effective theory, we show that
the mixing of the operator for the heavy-quark kinetic energy with the identity
operator is forbidden at the one-loop order by Lorentz invariance. This
explains why such a mixing was not observed in several one-loop calculations
using regularization schemes with a Lorentz-invariant UV regulator, and why no
UV renormalon singularity was found in the matrix elements of the kinetic
operator in the bubble approximation (the ``invisible renormalon''). On the
other hand, we show that the mixing is not protected in general by any
symmetry, and it indeed occurs at the two-loop order. This implies that the
parameter $\lambda_1^H$ of the heavy-quark effective theory is not directly a
physical quantity, but requires a non-perturbative subtraction.
