I am trying to enrich the event registration form (Attendee) with the partner_id from res.partner using a lookup over the email provided in the event registration and the one saved in res.partner.
To do this I am setting up an automated action which looks like this:

Model: Event Registration

Action: Update record

Trigger: On Creation

Field: Booked by (event.registration)

Evaluation Type: Python expression

Value: if event\.registration\.email\ ==\ res.partner.email:
record.res.partner.partner_id

Unfortunately it doesn't work properly and as soon as I activate it, the event registration over the website doesn't work anymore and gives an Error 500.
Maybe it is an easy one to solve.

Every help is highly appreciated.
Thanks
Nick