Setting `aboveskip` on inner environments to stretchable space fails to compile

[dcpurton]

The following MWE fails to compile (but works if aboveskip is not stretchable):

Same behaviour with belowskip.

\documentclass{article}
\usepackage{scripture}
\scripturesetup{poetry/aboveskip=0pt plus 2pt}
\begin{document}
\begin{scripture}
  Foo
  \begin{poetry}
    Foo
  \end{poetry}
  Foo
\end{scripture}
\end{document}