creating new command including siunitx - error due to spacing (?)

Question

I have created a new command for the glossarie-package. Especcially for symbols...

\newcommand*{\nsymbol}[4]{\newglossaryentry{#1}{
            type=symbols,
            name={#2},
            description={\nopostdesc},
            symbol={{#3}}, 
            user1={#4}, 
            sort={#1}}}

The main issue here (i guess) the following line works fine, but the second one gives me an 'Undefiened Control Sequence' and I can't determine whats the problem.

\nsymbol{abc}{alphabet}{$A_.{bc}$}{$\si{cm}$}
\nsymbol{abc}{alphabet}{$A_.{bc}$}{$\si{\metre}$}

I can only guess, but it seems the newcommand doesnt work quite well with siunitx, since the error says something like:

! Undefined control sequence. \@glo@useri ->$\si {\metre }$ l.38 ...alphabet}{$A_.{bc}$}{$\si{\metre}$}

Where there is a space between '\si' and '{\metre}', maybe this is the problem... It doesn't make any difference in, whether its mathmode or not. Thanks in advance, its probably stupid, but I would appreciate any helpful answer.

EDIT:that's kind of a minimal example, not quite sure, what's the problem here, so I copied a lot.

\documentclass{scrreprt}

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[locale=DE]{siunitx}
    \sisetup{detect-mode = false,
    detect-family=true,
    mode=math,
    %               output-decimal-marker={,},
    binary-units=true,
    group-separator={\,},group-minimum-digits=3%
}

        % upright indices-shortcut
\mathcode`\.="8000
\begingroup\lccode`~=`.
\lowercase{\endgroup\def~}#1{\mathrm{#1}}

\usepackage{hyperref}

\usepackage[acronym, symbols, nomain, toc]{glossaries}
\usepackage{booktabs}
\usepackage{tabu}

%%% new glossary styl
\newglossarystyle{symblongtabu}{
    \renewenvironment{theglossary}{
        \begin{longtabu}spread 0pt[l]{ccc<{\strut}l}
        }{
    \end{longtabu}}
    \renewcommand*{\glsgroupheading}[1]{}
    \renewcommand*{\glsgroupskip}{}
    \renewcommand*{\glossaryheader}{
        \toprule
        \bfseries Formelzeichen & \bfseries Bezeichnung &
        \bfseries Einheit & \bfseries Seite(n)
        \tabularnewline\midrule\endhead
        \bottomrule\endfoot}
    \renewcommand*{\glossentry}[2]{
        \glsentryitem{##1}
        % Entry number if required
        \glstarget{##1}{\glossentrysymbol{##1}} &
        \glossentryname{##1} &
        \glsentryuseri{##1} &
        ##2\tabularnewline}}

\makenoidxglossaries

%%% newsymb macro
\newcommand*{\nsymbol}[4]{\newglossaryentry{#1}{
        type=symbols,
        name={#2},
        description={\nopostdesc},
        symbol={{#3}}, 
        user1={#4}, 
        sort={#1}}}
\defglsentryfmt[symbols]{\ifmmode
    \glssymbol{\glslabel}
    \else
    \glsgenentryfmt~\glsentrysymbol{\glslabel}
    \fi}
%%% entries
\nsymbol{abc}{alphabet}{$A_.{bc}$}{$\si{cm}$}
\nsymbol{abc1}{alphabet1}{$A_.{bc1}$}{$\si{\metre}$}

\begin{document}

    \printnoidxglossary[type = symbols, style = symblongtabu]

    \gls{abc} \\
    \gls{abc1}

\end{document}

Answer 1

Just \protect it, as in \nsymbol{abc1}{alphabet1}{$A_.{bc1}$}{$\si{\protect\metre}$}

UPDATE: Nicola just provided the Rosetta Stone...adding \glsnoexpandfields to the preamble alleviates the need to protect the arguments. It works to "prevent expansion occurring when the entry's internal commands are set."

\documentclass{scrreprt}

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[locale=DE]{siunitx}
    \sisetup{detect-mode = false,
    detect-family=true,
    mode=math,
    %               output-decimal-marker={,},
    binary-units=true,
    group-separator={\,},group-minimum-digits=3%
}

        % upright indices-shortcut
\mathcode`\.="8000
\begingroup\lccode`~=`.
\lowercase{\endgroup\def~}#1{\mathrm{#1}}

\usepackage{hyperref}

\usepackage[acronym, symbols, nomain, toc]{glossaries}
\usepackage{booktabs}
\usepackage{tabu}

%%% new glossary styl
\newglossarystyle{symblongtabu}{
    \renewenvironment{theglossary}{
        \begin{longtabu}spread 0pt[l]{ccc<{\strut}l}
        }{
    \end{longtabu}}
    \renewcommand*{\glsgroupheading}[1]{}
    \renewcommand*{\glsgroupskip}{}
    \renewcommand*{\glossaryheader}{
        \toprule
        \bfseries Formelzeichen & \bfseries Bezeichnung &
        \bfseries Einheit & \bfseries Seite(n)
        \tabularnewline\midrule\endhead
        \bottomrule\endfoot}
    \renewcommand*{\glossentry}[2]{
        \glsentryitem{##1}
        % Entry number if required
        \glstarget{##1}{\glossentrysymbol{##1}} &
        \glossentryname{##1} &
        \glsentryuseri{##1} &
        ##2\tabularnewline}}

\makenoidxglossaries

%%% newsymb macro
\newcommand*{\nsymbol}[4]{\newglossaryentry{#1}{
        type=symbols,
        name={#2},
        description={\nopostdesc},
        symbol={{#3}}, 
        user1={#4}, 
        sort={#1}}}
\defglsentryfmt[symbols]{\ifmmode
    \glssymbol{\glslabel}
    \else
    \glsgenentryfmt~\glsentrysymbol{\glslabel}
    \fi}
%%% entries
\nsymbol{abc}{alphabet}{$A_.{bc}$}{$\si{cm}$}
\nsymbol{abc1}{alphabet1}{$A_.{bc1}$}{$\si{\protect\metre}$}

\begin{document}

    \printnoidxglossary[type = symbols, style = symblongtabu]

    \gls{abc} \\
    \gls{abc1}

\end{document}