How to add a decimal point in the units of a concentration?

Question

I have been writing my concentrations as per the code below using \SI{300}{\milli\gram\per\milli\litre} however my supervisor keeps writing a stupid . in all my feedback and wants the concentration to look like mg.ml$^-1$.

Everything I try writes the . as an accent on top of \ or the units. Is it possible to replace the space between the mg and ml with a dot?

Here is my minimal working example:

\documentclass[12pt]{article}
\usepackage{siunitx}
\begin{document}
\SI{300}{\milli\gram\per\milli\litre}
\end{document}