TikZ externalize MD5 sum error with XeLaTeX

Question

I've got some trouble with XeLaTeX and TikZ-externalize. I use the following setup.

\usetikzlibrary{external}                                                                                                                                                                                                   
\tikzset{external/system call={xelatex \tikzexternalcheckshellescape -halt-on-error -interaction=batchmode -jobname "\image" "\texsource"}}

Compiling is done with

xelatex -8bit -shell-escape

However I get the following warning:

! Package tikz Warning: The key 'up to date check=md5' is impossible,    there is 
no macro to compute MD5. Falling back to 'up to date check=diff'

This is only a warning, but if I misspell a command in my tikzpicture it uses the previously correct compiled pdf instead of throwing an error for my last change. With pdflatex and externalize this works perfects, it gives me the error and I can check that. So it must have to do with XeLaTeX. Linking md5sum to md5 doesn't resolved the problem.

This error is quite rare in the internet and I didn't find a solution for it.

Thanks,

Felix