[OS X TeX] Beamer Bibliograph problem

[Samuel Harry Cox, Jr.]

The following example has an error in the bibliography list when compiled with TeXShop.  The citation [CBD06] appears correctly in the first frame, but in the frame for the bibliography list it prints a symbol instead of [CBD06]. The same happens if I change the style to numeric.  
The analogous code for article document class rather than beamer works as expected.  I do not know what to do. Any ideas? 

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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