Problem when using grouping in choices (Bug #693)

\correctchoice and \wrongchoice are not taken into account if they are in a subgroup (braces) in a choices environment.
See minimal example below. The compilation fails, complaining about a missing \item.
Removing the braces makes the code compile.

Note that this problem does not seem to appear when using grouping in an AMCOpen macro.

\documentclass{article}
\usepackage{automultiplechoice}
\begin{document}
\begin{copieexamen}[1]

  \begin{question}{simple}
    Just a simple test question.

    \begin{choices}
      {
        \correctchoice{1}
        \wrongchoice{2}
      }
    \end{choices}
  \end{question}
\end{copieexamen}
\end{document}

For information, I encountered this bug while trying to generate many possible answers using a \foreach loop (from tikz), where I need to use braces, such as:

\foreach \ans in {1,2,...,18} {
  \ifnum\ans=3
     \correctchoice{\ans}
   \else
     \wrongchoice{\ans}
   \fi
}