Can't use a macro as argument to a tabular if it's not used in the first column

I have a more complex problem than the MWE I provide. The example, however, points directly to my issue.

I have a macro to do some processing (like specialline in the MWE) and pass it to another macro, which is responsible to produce the line of the itself (tableline).

documentclass[12pt,a4paper]{article}

newcounter{sum}
newcommand{tableline}[3]{%
    addtocounter{sum}{#3}%
    #1 & #2 & #3 
}

newcommand{specialline}[1][]{%
    defmyinput{#1}%
    ifxmyinputempty%
        defmytext{---}%
    else%
        defmytext{textit{myinput}}%
    fi%
    tableline{mytext}{}{100} %%% this works
    %tableline{}{mytext}{100} %%% this fails
}

begin{document}

begin{tabular}{ccc}
    A & B & C 
    hline
    tableline{X}{Y}{10}
    tableline{M}{L}{40}
    tableline{K}{T}{50}
    specialline
    specialline[something]
    hline
    & & thesum 
end{tabular}

end{document}

If the macro mytext goes to the first column, everything goes fine. But in any other column the compilation fails. Here’s the log:

! Undefined control sequence.
<argument> mytext 
                   
l.29        specialline
                   [something]
The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

The actual tableline and specialline I use are more complex and involves several other macros along with writing to a temporary file. So replicate part of the code of tableline in specialline is not desirable, since maintenance will suffer.

My question: why does it fails to compile and how can I overcome this problem?

documentclass[12pt,a4paper]{article}

newcounter{sum}

newcommand{tableline}[3]{%
    addtocounter{sum}{#3}%
    #1 & #2 & #3 
}

newcommand{specialline}[1][]{%
    tableline{}{ifrelaxdetokenize{#1}relax---elsetextit{#1}fi}{100}%
}

begin{document}

begin{tabular}{ccc}
    A & B & C 
    hline
    tableline{X}{Y}{10}
    tableline{M}{L}{40}
    tableline{K}{T}{50}
    specialline
    specialline[something]
    hline
    & & thesum
end{tabular}

end{document}