为什么 tikzmarknode 无法识别？

Question

\documentclass{article}

\usepackage{xcolor}

\usepackage{tikz}

\usetikzlibrary{tikzmark,shapes.geometric}


\begin{document}


\tikzset{tikzmark prefix=ex5-}
Putting a node around \tikzmarknode{txt}{some text} means we can
connect text together, including in maths:
\[
\tikzmarknode{a}{\sum_{k=1}^n} k^{\tikzmarknode{b}{2}}
\]
\begin{tikzpicture}[remember picture,overlay]
\draw[->] (txt) -- (a);
\draw[->] (a.south) to[out=-90,in=-45] (b.south east);
\end{tikzpicture}


\end{document}

我得到了 tikzmarknode 未定义的控制序列。