Missing number, treated as zero. \end{tabu}

错误代码
%\usepackage{colortbl}
%\usepackage{tabu}
\begin{tabu}{|X|X|}
\begin{tabu}{|X|X|}
a & c
\end{tabu}
& b \\
\end{tabu}
解决方式
%\usepackage{colortbl}
%\usepackage{tabu}
\newsavebox\grumble\savebox\grumble
{\begin{tabu}{|X|X|}
		a & c
\end{tabu}}

\begin{tabu}{|X|X|}
	\usebox{\grumble} & b \\
\end{tabu}	