I updated MikTex 2.9 Monday, and now can't compile while loading 'mhchem'. I'm using pdflatex [Version 3.14159265-2.6-1.40.16 (MiKTeX 2.9 64-bit)] to compile the following:
\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}
Hello, world!
\end{document}
throws the following error:
! LaTeX error: "kernel/command-not-defined"
! Control sequence \prop_get:Nn undefined.
! See the LaTeX3 documentation for further information.
! For immediate help type H <return>.
l.180 ..._variant:Nn \prop_get:Nn { cV }
| This is a coding error.
| LaTeX has been asked to use a control sequence '\prop_get:Nn': this has not been defined yet.
\listfiles
to the preamble and post the resulting list from the end of the log file.