What's causing this unexpected behavior of the cleveref package, which occurs when I try to reference multiple things at a time?

I'm trying to reference two propositions together, in a document that I'm typesetting. From the cleveref documentation, I learned that \cref{label1,label2} is the correct syntax for doing so, but I'm running into trouble when I try. Here is a minimal working example:

\documentclass{article}
\usepackage{amsthm,cleveref}
\newtheorem{prop}{Proposition}

\begin{document}

\begin{prop}\label{a}
\end{prop}
\begin{prop}\label{b}
\end{prop}

\cref{a} \cref{b}\\

\cref{a,b}

\end{document}


The output is just fine for the single references, \cref{a} and \cref{b}, but \cref{a,b} produces an error message:

cref reference format for label type prop' undefined

I find this strange, because everything works fine for single references. Am I messing something up, or is this a bug?

You need to tell cleveref what the plural name is:

\crefname{prop}{Proposition}{Propositions}


As a matter of good practice, you might also specify

\Crefname{prop}{Proposition}{Propositions}


so that \Cref and friends work as expected. Adjust as you see fit.

Here's a complete MWE:

% arara: pdflatex
% arara: pdflatex
\documentclass{article}
\usepackage{amsthm,cleveref}
\newtheorem{prop}{Proposition}
\crefname{prop}{Proposition}{Propositions}%<--- new bit!
\Crefname{prop}{Proposition}{Propositions}%<--- new bit!
\begin{document}

\begin{prop}\label{a}
\end{prop}
\begin{prop}\label{b}
\end{prop}

\cref{a} \cref{b}

\cref{a,b}

\end{document}
`