[Tex/LaTex] Usage of titlesec package when numbering sections

numberingsectioningtitlesec

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?

Best Answer

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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