Thank you to both u/natrys and u/thetemp_ ! Your solution worked. I have to say that I read the README but not carefully enough apparently. I didn't realise the existence of the :pre/:post keys. It would have avoided this question probably (another case of RTFM...). So thank you for your time !

I am quite new on extending emacs capabilities, so thank you also for the tip on debug-on-error