TikZ：坐标计算的编译器错误，计算库

Question

\documentclass{standalone}

\usepackage{tikz,pgf}

\usetikzlibrary{decorations.markings,%
    snakes,fit,scopes,arrows,calc,shapes.misc,positioning,%
    shapes.arrows,chains,matrix,positioning,decorations.pathmorphing,shapes,backgrounds,decorations.text}

\begin{document}

\tikzset{
    fuzzy/.style = {decorate, decoration = {random steps, segment length = 0.5mm, amplitude = 0.15pt}},
    ns/.style = {rectangle,minimum size = 6mm,rounded corners = 3mm,very thick,draw,font = \ttfamily,fuzzy}
}

\newcommand{\OL}[1]{{$\overline{x}_{#1}$}}
\newcommand{\NL}[1]{{${x}_{#1}$}}

\begin{tikzpicture} [
        >=latex, thick, 
        ns/.style = {rectangle, draw, fill = white, fuzzy, rounded corners = 3mm},
        decision/.style = {rectangle, draw, fill = gray, fuzzy, rounded corners = 3mm},
    ]
    \ttfamily
    \begin{scope}
        \coordinate node[decision] (nx12) at (0,0) {\OL{12}};
        \coordinate node[decision] (x5) at ($(nx12)+(1,1)$) {\NL{5}};
        \coordinate node[decision] (nx8) at ($(nx12)+(1,-1)$) {\OL{8}};

        \coordinate node[ns] (x2) at ($(x5)+(2,1)$) {\NL{2}};
        \coordinate node[ns] (nx7) at ($(nx12)+(2,-.5)$) {\OL{7}};
        \coordinate node[ns] (x4) at ($(x5)+(4,1)$) {\NL{4}};
        %\coordinate node[ns] (nx13) at ($(x4)+(1,1)$) {\OL{13}};
        %\coordinate node[ns] (x13) at ($(x4)+(1,-1)$) {\NL{13}};
    \end{scope}

\end{tikzpicture}

\end{document}

如果有人评论说x4一切进展顺利，但现在却是“<...>必须有一个（可能是空的）标签<...>”