titlesec error "undefined control sequence"

Question

I'm new to titlesec and LaTeX in general, trying to use it to achieve chapters in a {book} class that are medium-sized and on the same line, viz:

Chapter 1: An Unlikely Event

Here's a minimized example:

\documentclass[english,12pt,oneside]{book}
\usepackage{titlesec}
\titleformat{\chapter}[hang]{\normalfont\medium\bfseries}{\chaptertitlename\ \thechapter:}{1em}{}
\begin{document}

\chapter{An Unlikely Event}

\end{document}

The error that returns is:

ERROR: Undefined control sequence.

--- TeX said ---
<argument> \normalfont \medium 
                               \bfseries 
l.77 \chapter{An Unlikely Event}

--- HELP ---
TeX encountered an unknown command name. You probably misspelled the
name ...

I think this is probably a simple error on my part, but I'm too young to know what it is.

Answer 1

you have used the command \medium, which doesn't exist. what you probably meant is \mdseries, which is the default.

since you responded in a comment that what you really want to do is change the size of the font, the command for that is \normalsize.