buttonfg option gives error

I am not a regular animate user so forgive me if I'm missing something, but is

\documentclass{article}
\usepackage[buttonfg=0.5]{animate}
\begin{document}
abc
\end{document}

supposed to produce an error? I get

Runaway argument?
0.5:\@nil \global \let \@anim@@fgcolour \@anim@colour \ifnum \if@anim@dvips \ET
C.
! Paragraph ended before \@anim@@colours was complete.
<to be read again> 
                   \par 
l.3626 
       
? 
! Emergency stop.
<to be read again> 
                   \par 
l.3626