MailChimp Extended Plugin: update subscriber

Hi there,
I came across a new problem.

So far:
new app user via the app gets “subscribed” to a mailchimp list. Then gets automatically tagged in a flow over there. Done.

Now:
a new app user has already subscribed to our mailchimp list via our website. So what I want is to “subscribe or update” instead of just “subscribe”

I want to “subscribe if new” but update first name + last name, if existing and also add a new tag “prototype”

So I created this:

But I keep getting this from my users. I just can’t figure out what I’m doing wrong:

Please help. What am I doing wrong?