overlay vs. counter

Hello,

I have a problem creating some slides:
I want to use the overlay function, but then all counter values appear with 
zero. If I don't use the overlay function, the example works proper.
Can anybody help me?

Thanks,
Markus

===Begin expample===

\documentclass[pdf, slideColor]{prosper}
\usepackage[english]{babel}
\usepackage{amsmath}

\newtheorem{definition}{Definition}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}[definition]{Lemma}
\newtheorem{example}[definition]{Example}
\newtheorem{corollary}[definition]{Corollary}
\newtheorem{remark}[definition]{Remark}


\begin{document}

\overlays{4}{%
\begin{slide}{1st slide}
	\begin{itemstep}
		\item 1st item
		\item 2nd item
		\item 3rd item
		\item 4th item
	\end{itemstep}
\end{slide}
}

\begin{slide}{2nd slide}
	\begin{alignat}{2}
	\sum_{k=1}^n&=1+2+\ldots+n\\
	&=\frac{n(n+1)}{2}
	\end{alignat}
\end{slide}

\begin{slide}{Burke's Theorem}
\begin{theorem}
1st theorem
\end{theorem}
\begin{remark}
1st remark
\end{remark}
\begin{remark}
2nd remark
\end{remark}
\end{slide}

\end{document}




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