Changing background color leaves white lines above and below title

\documentclass[handout]{beamer}
%\usepackage{color} beamer loads xcolor anyway
\usecolortheme{rose}
\useoutertheme{infolines} % header and footer
\setbeamertemplate{headline}{} % no header line
\begin{document}
 \begin{frame}
  \frametitle{slide 1}
  first
 \end{frame}
{
\setbeamercolor{normal text}{bg=yellow!20}
 \begin{frame}
  \frametitle{slide 2}
  second
 \end{frame}
}
\end{document}