[Misiur]

This comment (especially the pdf excerpt) will explain as to why this error pops up. You can solve it like:

pawn Code:

if (numargs() == 1)
    {
        strcat((string[0] = EOS, string), msg);
        return string;
    }

(This is macroless strcpy)

There is other way using macros, but detecting single parameter is way more difficult than detecting none or more than 1.