Simply, radianpersecond doesn't work; all other units does.

\documentclass[twoside]{article}
\usepackage{amsmath}
\usepackage{commath}
\usepackage{siunitx}
\begin{document}
\begin{align}
a&=\SI{1}{cm}\\
\end{align}
\end{document}


The error:

Undefined control sequence. \end{align}


I don't understand why only this unit doesn't work!

• This means that \radianpersecond is not defined! Shouldn't it be \radian\per\second? – egreg Nov 13 '14 at 17:42
• OK Thanks that works, the thing is that TeXStudio gave me that option: \radianpersecond so I thought that's the one! – student1 Nov 13 '14 at 17:51
• Well, that's TeXStudio's fault. – egreg Nov 13 '14 at 17:53

The package doesn't define \radianpersecond. Either you type

\SI{1}{\radian\per\second}


\DeclareSIUnit\radianpersecond{\radian\per\second}


and then you'll be allowed to type also

\SI{1}{\radianpersecond}


Note: it seems a bug in TeXStudio if its palettes suggest \radianpersecond as an existing unit.

• Even though you could, I would recommend not creating a macro for that (not saying that you are). You save two backslash keystrokes and you make your code less portable. – Matthew Leingang Nov 13 '14 at 18:42