Center reaction with chemfig: subscript causes shifting

Question

As can be seen, the second molecule of the reagents is not aligned with the first, probably due to the presence of the methane subscript 4. I tried this solution, but it doesn't work. Any idea?

\documentclass{article}
\usepackage{chemfig}
\begin{document}
\schemestart
\setchemfig{debug=true}
\chemfig{C@{a}H_4} \+ \arrow(@a--@b){0} \chemfig{@{b}-[,.6,,,dotted]M-M-M-[,.6,,,dotted]} 
\arrow 
\chemfig{-[,.6,,,dotted]M(-[2]CH_3)-M(-[2]H)-M-[,.6,,,dotted]}
\schemestop
\end{document}

Answer 1

\documentclass{article}
\usepackage{chemfig}
\begin{document}
\schemestart
\chemfig{CH_4}
\+
\chemfig{\vphantom{C}-[,.6,,,dotted]M-M-M-[,.6,,,dotted]}
\arrow
\chemfig{-[,.6,,,dotted]M(-[2]CH_3)-M(-[2]H)-M-[,.6,,,dotted]}
\schemestop
\end{document}

or

\documentclass{article}
\usepackage{chemfig}
\begin{document}
\schemestart
\chemfig{CH_4}
\+
\chemfig{M(-[4,.6,,,dotted])-M-M-[,.6,,,dotted]}
\arrow
\chemfig{-[,.6,,,dotted]M(-[2]CH_3)-M(-[2]H)-M-[,.6,,,dotted]}
\schemestop
\end{document}