acronym package - acroplural not working in acronym environment

Question

I'm trying to use the acronym environment from the acronym package to typeset a list of acronyms in my document. I want to specify a non-standard plural form for an acronym using \acroplural (which, according to the package documentation, "is meant to be used in the acronym environment"), but doing so is giving me an error message.

MWE

Document

\documentclass{article}
\usepackage{acronym,hyperref}
\begin{document}
\begin{acronym}
    \acro{POI}{point of interest} \acroplural{POI}{POIs}{points of interest}
\end{acronym}
\end{document}

Error Messages

LaTeX Warning: Hyper reference `acro:POI' on page 1 undefined on input line 8.
LaTeX Warning: Hyper reference `acro:POI' on page 1 undefined on input line 8.
document.tex:8: Undefined control sequence.
\hyper@@link ->\let \Hy@reserved@a 
                                   \relax @ifnextchar [{\hyper@link@ }{\hyp...
l.8 ...}{point of interest} \acroplural{POI}{POIs}
                                                  {points of interest}
document.tex:8: Argument of \AC@acroplurali has an extra }.
<inserted text> 
                \par 
l.8 ...}{point of interest} \acroplural{POI}{POIs}
                                                  {points of interest}
Runaway argument?
{POI}{\hyper@link@ }\def \reserved@b {\hyper@link@ [link]}\futurelet \ETC.
document.tex:8: Paragraph ended before \AC@acroplurali was complete.
<to be read again> 
                   \par 
l.8 ...}{point of interest} \acroplural{POI}{POIs}
                                                  {points of interest}
[1] (./document.aux)

Related

How to pluralize an acronym which ends in 'S' correctly? is a similar question, but it asks about defining a non-standard plural form outside of the acronym environment.

Answer 1

Omitting hyperref your code compiles but inserts a spurious points of interest, it would also lead \aclp{POI} to give POIs rather than points of interest.

This is because the second argument of \acroplural (the short-form plural) is an optional argument and if supplied should be supplied as \acroplural{POI}[POIs]{points of interest}. From the package documentation:

In this case it is not necessary to supply it as only the long-form plural is not given by appending an s. This issue is not specific to \acroplural or the acronym environment, it would also occur in \newacroplural and \acrodefplural which use the same syntax.

\documentclass{article}
\usepackage{acronym}
\begin{document}
\begin{acronym}
    \acro{POI}{point of interest} 
    \acroplural{POI}{points of interest}
\end{acronym}
\end{document}

However with hyperref there is a second issue if the optional argument is not supplied

\documentclass{article}
\usepackage{acronym}
\usepackage{hyperref}
\begin{document}
\begin{acronym}
    \acro{POI}{point of interest} 
    \acroplural{POI}{points of interest}
\end{acronym}
\end{document}

giving the error

! Undefined control sequence.
\hyper@@link ->\let \Hy@reserved@a 
                                   \relax \@ifnextchar [{\hyper@link@ }{\hyp...
l.8     \acroplural{POI}{points of interest}

This can be resolved as you noted in the comments by explicitly supplying the optional argument as \acroplural{POI}[POIs]{points of interest}. However the documentation of acronym seems quite clear that this should not be necessary which would make this a bug in the package.

As \acroplural{POI}{POIs}{points of interest} is parsed as giving POI and POIs as the acronym and acronym long-form plural, and then just having the group {points of interest} following it (this is not swallowed as an argument as you can see by omitting hyperref). You therefore end up seeing the more severe hyperref-related error even though the first mistake was in syntax of \acroplural.