Possible Duplicate:
Badbox from PGF layer using fit
This peace of code is compiled with 'Underfull \hbox (badness 1484) in paragraph at lines 23--23' message. Line 23 is \node[draw, inner sep = 10pt, fit=(MUL1) (MUL2)] {};. How can I get rid of underfull box?
\documentclass[a4paper]{report}
\usepackage{tikz}
\usetikzlibrary{positioning,fit}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
\begin{document}
\begin{tikzpicture}
\node[draw] (MUL1) {MUL1};
\node[draw, right = of MUL1] (MUL2) {MUL2};
\node[left = of MUL1] (A) {A};
\node[right = of MUL2] (B) {B};
\draw[->] (A) -- (MUL1);
\draw[->] (MUL1) -- (MUL2);
\draw[->] (MUL2) -- (B);
\begin{pgfonlayer}{background}
\node[draw, inner sep = 10pt, fit=(MUL1) (MUL2)] {};
\end{pgfonlayer}
\end{tikzpicture}
\end{document}