Making Vpp and Vrms units in LaTeX with siunitx

The siunitx package for LaTeX makes it easy to handle units in both normal and math modes. I end up needing units like V_rms when talking about electronics, so I add this in my preamble:

\usepackage{siunitx}

\DeclareSIUnit \vrms {\ensuremath{\mathrm{V_{rms}}}}
\DeclareSIUnit \vpp {\ensuremath{\mathrm{V_{pp}}}}

...to define these units to siunitx. I can then invoke the units with something like

free air.  I drove them to about \SI{9}{\vpp}.

where I know siunitx will then make the proper space between the number and the unit. This all renders like this:

...in the PDF output.