mhchem ver.4 not working after update! Control sequence undefined

Question

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.

Answer 1

You have a local old copy of chemgreek masking your installed one:

(C:\Users\TobiasGram\AppData\Roaming\MiKTeX\2.9\tex\latex\chemgreek\chemgreek.sty

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

("C:\Program Files\MiKTeX 2.9\tex\latex\l3packages\xparse\xparse.sty"
Package: xparse 2016/01/19 v6377 L3 Experimental document command parser
\l__xparse_current_arg_int=\count170
\l__xparse_m_args_int=\count171
\l__xparse_mandatory_args_int=\count172
\l__xparse_processor_int=\count173
\l__xparse_v_nesting_int=\count174
)
Package: chemgreek 2015/04/09 v1.0 interface for upright greek letters for use in chemistry (CN)

It should be more like

(/usr/local/texlive/2015/texmf-dist/tex/latex/chemgreek/chemgreek.sty
(/home/davidc/texmf/tex/latex/l3packages/xparse/xparse.sty
Package: xparse 2016/01/19 v6377 L3 Experimental document command parser
\l__xparse_current_arg_int=\count170
\l__xparse_m_args_int=\count171
\l__xparse_mandatory_args_int=\count172
\l__xparse_processor_int=\count173
\l__xparse_v_nesting_int=\count174
)
Package: chemgreek 2016/02/10 v1.0e interfaceforuprightgreeklettersforuseinchem
istry (CN)