# ntheorem not placing marker on next line

Hi does anyone have a solution to this:

\documentclass[12pt,reqno,a4paper,draft]{article}
\usepackage[margin=3.cm,heightrounded]{geometry}
\usepackage{mathtools,amsmath,amssymb}
\usepackage[amsmath,thmmarks]{ntheorem}
\theoremseparator{.}
\theoremstyle{nonumberplain}
\theorembodyfont{\normalfont}
\theoremsymbol{\enspace\ensuremath{\Box}}
\newtheorem{proof}{Proof}
\begin{document}
\begin{proof}
Blah
\begin{equation*}
blah
\end{equation*}
which always hold a an normal almost contact
metric structure (see [1, Lemma~2.1]).
\end{proof}
\end{document}


as can be seen the box overflows the line instead of being moved to the next line. The text one the last line span the full text width.

-

\theoremsymbol{\nolinebreak[1]\hspace*{.5em plus 1fill}\ensuremath{\Box}}