\setcmpdlabel prevents initial compilation

[matterhorn103]

MWE:

\documentclass[11pt]{article}
\usepackage{chemnum}

\begin{document}
\cmpd*{MJM-450} \setcmpdlabel{MJM-450}{1b}
\cmpd{MJM-450}  % Fails whether this line is present or not
\end{document}

When compiling for the first time you get the error:

! Argument of \__prop_map_tokens:nw has an extra }.
<inserted text> 
                \par 
l.5 \cmpd*{MJM-450} \setcmpdlabel{MJM-450}{1b}
                                              
?

Answering with r results in successful compilation and on future occasions there are no further complaints.