siunitx and missing decimal point

The command \si{units} is for units only. You have to use the \SI{number}{units} for what you want to do.

I guess i am replying really late for your query, but you should use \num directly and not nest it in \si.

For a number

\num{12.5}

and \SI (in upper case) for a number with a unit

\SI{12.5}{\milli\gram}