[OS X TeX] Bibliography problem with beamer

[Samuel Harry Cox, Jr.]

This example an error in the bibliography list.  It correctly shows the citation [CBD06] in the first frame, but in the frame for the bibliography list it print a symbol instead of [CBD06]. The same if I change the style to numeric. I use TeXShop. Here is the latex and after my output.

I need some help, please.  Oh, the analogous code for article document class rather than beamer works as expected.

Sam

\documentclass{beamer}
%\documentclass{article}
\usepackage[babel]{csquotes}
\usepackage[style=alphabetic]{biblatex}
%settings 
\mode<presentation>{%set beamer mode
    \usetheme{Warsaw}
}

\bibliography{unified}
%document
\begin{document}
%------------------------------------------------------------------------------------------
\begin{frame}
 In frame citation:   \cite{cairns06}
\end{frame}
%------------------------------------------------------------------------------------------
\begin{frame}
    \printbibliography
\end{frame}
%------------------------------------------------------------------------------------------
\end{document}

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