We revisit size controllability results in P\"otscher and Preinerstorfer (2021) concerning heteroskedasticity robust test statistics in regression models. For the special, but important, case of testing a single restriction (e.g., a zero restriction on a single coefficient), we povide a necessary and sufficient condition for size controllability, whereas the condition in P\"otscher and Preinerstorfer (2021) is, in general, only sufficient (even in the case of testing a single restriction).