Beamer 标题带框架标题

Question

让我们考虑一下这个 MWE

\documentclass{beamer}

\makeatletter
\setbeamertemplate{frametitle}{}
\setbeamertemplate{headline}
{
  \leavevmode
  \begin{beamercolorbox}[wd=\paperwidth,ht=4.8ex,dp=2ex,left]{section in head}
    \hspace{1ex}{\insertsection} -- {\insertframetitle}
  \end{beamercolorbox}
}
\makeatother

\begin{document}

\section{Introduction}
\begin{frame}
  \frametitle{One}
  Hello
\end{frame}

\begin{frame}
  \frametitle{Two}
  World
\end{frame}

\end{document}

排版这个，错误

! Undefined control sequence.
\beamer@@tmpl@headline ...} -- {\insertframetitle 
                                                  } \end {beamercolorbox} 
l.14 \begin{document}

? 

有解决办法吗？