# Underfull \hbox (badness 10000) detected at line XX []$[]$

I know there were a lot of questions regarding underfull hbox, but in my case error

Underfull \hbox (badness 10000) detected at line 5
[]$[]$


point to line

5: \begin{document}


How can hbox be underfull if compilation of the document has not even started?

EDIT: after inspection I have found out that autonum package is creating this error.

\documentclass{article}

\usepackage{autonum}

\begin{document}
\end{document}

-

It is a bug in package autonum, as @Heiko Oberdiek mentioned in his answer in question autonum-does-not-work-with-eqnarray.
Please report the bug to the maintainer of autonum.