  To quantify the theory error on $R_K$, essentially means to quantify the
uncertainty due to QED corrections since the latter breaks lepton flavour
universality through the lepton masses. Since experiment uses photon shower
programs, e.g. \texttt{PHOTOS}, to capture QED effects, assessing the
uncertainty involves investigating effects not captured by the specific use of
these tools. This includes structure-dependent corrections, potentially large
non-logarithmic terms and charmonium resonances entering the lower bin by
migration of radiation. We are able to close in on these loopholes. For
example, using gauge invariance, we show that structure-dependent QED
corrections do not lead to additional (sizeable) hard-collinear logs of the
form ${\cal O}(\alpha) \ln m_\ell/m_B$. Hence, from the theory point of view
$R_K$ is a safe observable.
