! Missing \endcsname inserted while using showkeys

Question

Using showkeys of my MikTeX 2.9, the code below

\documentclass{article}
\usepackage{showkeys}
\begin{document}
    section \ref{sec:methods} section \ref{methods}
    \section{Methods} 
    \label{sec:methods}
\end{document}

yields weird results:

As you see, only the first reference is detected by the package, and I get

! Missing \endcsname inserted.
<to be read again> 
                   \BooleanFalse 
l.20    section \ref{sec:methods} section \ref{
                                             methods}
The control sequence marked <to be read again> should
not appear between \csname and \endcsname.
LaTeX Warning: Reference `\endcsname ' on page 1 undefined on input line 20.

My file does not include any non-UTF character. What's the problem here?

Answer 1

In a correctly installed LaTeX you should get no error from the posted example, however if you use the optional argument of \usepackage to force loading an old version with a new latex, you do get the error shown.

\documentclass{article}
\usepackage{showkeys}[=2014-10-28]
\begin{document}
    section \ref{sec:methods} section \ref{methods}
    \section{Methods} 
    \label{sec:methods}
\end{document}

Produces

! Missing \endcsname inserted.
<to be read again> 
                   \BooleanFalse 
l.6     section \ref{sec:methods} section \ref{
                                               methods}
? 

So it appears that you have an old showkeys in your input path.