Usage of titlesec package when numbering sections

This question evolved from my earlier question posted here. It turnt out that what I gave as the MWE actually worked, so I investigated a bit and found the real problem. I have a document in which I generally do not want to number the chapters, sections and subsections. Only one section should contain roman numbered subsections. Considering the following code:

\documentclass{report}
\usepackage{titlesec}
\titleformat{\subsection}{\normalsize\bf\it}{}{}{}
\setcounter{secnumdepth}{-1}
\begin{document}
\tableofcontents
\chapter{mwe}
\section{unnumbered subsections}
\subsection{unnumbered}
\section{roman numbered subsections}
\setcounter{secnumdepth}{2}
\renewcommand{\thesubsection}{\textnormal{\roman{subsection}}.}
\subsection{roman one}
\subsection{roman two}
\setcounter{secnumdepth}{-1}
\chapter{Second chapter}
\section{unnumbered subsections}
\subsection{unnumbered}
\end{document}

which is the solution from my earlier post adding the titlesec package and modifying the subsection's title (i.e. lines 2&3). Here I get the 'Missing number treating as zero' error at the line '\subsection{roman one}'. Can anybody tell me why and a fix or work-around?

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Could you expand your question a bit please, so that users will be able to tell what's the deal here without having to compile your MWE or to go to that other question? Ideally, you'd also find a title more specific than "problem with X". –  doncherry Nov 1 '11 at 12:16

The fourth mandatory argument of \titleformat must not be empty; you can use

\titleformat{\subsection}{\normalsize\bfseries\itshape}{}{0pt}{}

Taking into account a comment, perhaps you're trying to do something like this

\titleformat{\subsection}{\normalsize\bfseries\itshape}{\textnormal{\roman{subsection}.}}{1em}{}
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That was a quick answer, thanks. However, when I change the code accordingly the roman numbering in the document - not in the toc - disappears...any ideas? –  D.Roepo Oct 31 '11 at 16:46
Good! @user7091: also note Gonzalo used the better newer font switches. \it overrides the earlier \bf. –  Stefan Kottwitz Oct 31 '11 at 16:47
@user7091: see my updated answer. –  Gonzalo Medina Oct 31 '11 at 16:51