Problem with fancyvrb and optional environment argument

I have the following sample code where I use Verbatim within an
environment with an optional argument:

\documentclass{book}
\usepackage{fancyvrb}
\newenvironment{dialogue}[1][Label]{
#1
\VerbatimEnvironment
\begin{Verbatim}
}
{\end{Verbatim}}

\begin{document}
\begin{dialogue}
Line 1
Line 2
\end{dialogue}
\end{document}

This fails with:
! FancyVerb Error:
Extraneous input `Line 1' between \begin{dialogue}[<key=value>] and
line end
..
\FV@Error ... {FancyVerb Error:
\space \space #1
}

If I change the call to \begin{dialogue}[Label] then the code works.
I've tried tracing macros, but I can't figure out what's wrong.

Anyone else have an idea why the optional argument breaks the code?

.

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