Can't get rid of strange underfull \hbox with TikZ fit library [duplicate]

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}

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marked as duplicate by Jake, Guido, Thorsten, percusse, Loop SpaceDec 3 '12 at 10:01

Welcome to TeX.sx! –  Kurt Dec 3 '12 at 2:13
This looks like it probably is a duplicate of the question that Jake points to. It currently has 4 votes to close as a duplicate. Could someone with the right version of TikZ (I have the CVS) please verify that the solution there does solve this one before anyone casts the final vote-to-close as duplicate? –  Loop Space Dec 3 '12 at 9:55
(Someone just verified it for me in chat. I shan't name David Carlisle as he doesn't want it known that he has compiled a document using TikZ. Anyway, confirmed as duplicate.) –  Loop Space Dec 3 '12 at 10:01

It has something to do with the empty node.

Writing

\begin{pgfonlayer}{background}
\node[draw, inner sep = 10pt, fit=(MUL1) (MUL2)] {\hspace*{\fill}~};
\end{pgfonlayer}%


instead will get rid of the underfull error. Seems there's got to be a better way to achieve your effect.

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There is a bug in the fit library of TikZ/pgf (v2.10). With the latest CVS version of TikZ/pgf (cf. my answer to How to install a current version of TikZ?), your MWE compiles without error or warning.

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