I need to change \thechapter into something that spans several characters. Doing so mixes up characters in my customized table of contents (as well as the ordinary ToC). Here's an example:

Here's an MWE:
\documentclass{book}
\usepackage{titlesec}
\usepackage{titletoc}
\titleformat{\chapter}{\huge\bfseries}{\chaptertitlename~\thechapter.}{1ex}{\huge}
[%
\vspace*{1em}
\startcontents
\normalfont\normalsize\printcontents{}{0}{\setcounter{tocdepth}{1}}%
]
\begin{document}
\renewcommand\thechapter{abcdef}
\chapter{Intro}
\section{Preliminaries}
\section{Goals}
\section{Formulas}
\end{document}
How can I increase the space reserved for section counter in the ToC, so that it does not clash into the section name?
