\forestmathsetlength looses sign if length starts with a zero

[u-fischer]

In the following example, -0.5mm gives a positive length. If one force forest to use the pgf commands with \forestmathsetlength \blub {-0.5mm+0cm} then the value is correct again.

\documentclass{article}
\usepackage{forest}
\begin{document}
\newlength\blub
\forestmathsetlength \blub {-5mm} 
\showthe\blub %negative

\forestmathsetlength \blub {-0.5mm} 
\showthe\blub %positive
\end{document}

> -14.22636pt.
l.226 \showthe\blub
                    %negative
? 
> 1.42262pt.
l.230 \showthe\blub
                    %positive

The problem is imho that forest is storing the integer part in a count, and if the value is zero the sign is lost (-1.5mm would work fine).