Beamer 演示和编译器错误 - ！缺少} 插入。} \end{frame}

Question

我的编译器标记了一个错误，但一切似乎都正常工作。有人能帮我理解这个问题吗？

\documentclass{beamer}
\usetheme[progressbar=frametitle]{metropolis}
\setbeamertemplate{frame numbering}[fraction]
\useoutertheme{metropolis}
\useinnertheme{metropolis}
%\usefonttheme{metropolis}
\usecolortheme{spruce}
\setbeamercolor{background canvas}{bg=white}
\usepackage{kpfonts}
\usepackage{amsmath}
\definecolor{mygreen}{rgb}{.125,.5,.25}
\title{Understanding the MIDI Function}
\usepackage{cases}
\date{}
\begin{document}
\metroset{block=fill}

\begin{frame}
\titlepage
\end{frame}

\begin{frame}[t]{Examining the Function} \vspace{10pt}
\begin{block}{MIDI Function}
\begin{align*}

\mathsf{f(s) &= 440 \cdot 2^{\frac{(s-29)}{12}}}\\
\text{$s$ is the MIDI Number.}\\
\mathsf{f(s) &= 440 \cdot e^{(s-29)} \cdot \frac{\log{2}}{12}}

\end{align*}
\end{block}
\vspace{10pt}

\vspace{10pt}
\begin{block}{MIDI Number}

  \[ \mathsf{s=\frac{12}{\sqrt{2}} \cdot \log\left(\frac{f}{440}\right)+29}\]

\end{block}

\end{frame} < - error ! Missing } inserted. <inserted text>} \end{frame}


\end{document}