»missing \item« despite using enumitem's resume

Question

The following MWE does not work as expected: It throws an error during compilation, despite the result seems to be correct. MWE:

\documentclass[11pt]{article} % use larger type; default would be 10pt

\usepackage[utf8]{inputenc} % set input encoding (not needed with XeLaTeX)

\usepackage{enumitem}

\title{Brief Article}
\author{The Author}

\begin{document}
\maketitle

\section{First section}

\begin{enumerate}
\item[i.] first item
\begin{enumerate}
\item nested item under first item
\end{enumerate}
\item[i + 1.]  second item
\end{enumerate}
commentary
\begin{enumerate}[resume]
\begin{enumerate}[resume]
\item shall be nested item under second item
\end{enumerate}
\item[j] third item
\end{enumerate}

\end{document}

The thrown error's message is "Something wrong, perhaps a missing \item [at] l.24 \begin{enumerate}[resume]". But this message should not appear since I'm using enumitem.

The result seems (!) to be correct -- I don't know exactly because of a missing counter before the (a) in the second list (2.(a)):

Besides: How could I achieve a numbering (.() (i. e. 2.(a)?

Answer 1

You could add an empty \item[]:

\documentclass[11pt]{article} % use larger type; default would be 10pt

\usepackage[utf8]{inputenc} % set input encoding (not needed with XeLaTeX)

\usepackage{enumitem}

\title{Brief Article}
\author{The Author}

\begin{document}
\maketitle

\section{First section}

\begin{enumerate}
\item[i.] first item
\begin{enumerate}
\item nested item under first item
\end{enumerate}
\item[i + 1.]  second item
\end{enumerate}
commentary
\begin{enumerate}[resume]
\item[] %<..............
\begin{enumerate}[resume]
\item shall be nested item under second item
\end{enumerate}
\item[j] third item
\end{enumerate}

\end{document}