Tikz 图形的圆圈在绘图中产生奇怪的数字

Question

我正在制作一些网络图，想建立一个星型拓扑图。我不知道中间的圆圈和它的数字来自哪里：

\usepackage{tikz}
  \usetikzlibrary{%
  graphs,
  graphs.standard,
  }
\begin{document}
      \tikz \graph [simple, nodes={circle,draw}] {
        subgraph K_n [n=6, clockwise];  % Get rid of the following edges: 
        1 -!- 2; 1 -!- 3; 1 -!- 5; 1 -!- 6  
        2 -!- 3; 2 -!- 4; 2 -!- 6;
        3 -!- 2; 3 -!- 4; 3 -!- 5;
        4 -!- 5; 4 -!- 6;
        5 -!- 6;
        6 -!- 1;
        }; 
\end{document}