Cashier and Stripe again. You passed an empty string for 'items[0][price]'

Is it just me or is the whole Stripe/Cashier caper quite fragile? I thought I had it all working a while ago and have come back and it's broken. After a few days of debugging I have upgraded everything. Cashier is now 14.2.1 and the Stripe API is at the recommended version of 2022-08-01. My code is basically identical to the example in the Cashier docs, which is the same as the Andre Madarang videos on here. I can't find anything weird. Yet this line

$user->newSubscription($plan->identifier, $plan->stripe_id)
             ->create($request->paymentMethod);

to create the actual subscription, just fails everytime, with the exception,

You passed an empty string for 'items[0][price]'. We assume empty values are an attempt to unset a parameter; however 'items[0][price]' cannot be unset. You should remove 'items[0][price]' from your request or supply a non-empty value.

which is a Stripe invalid request exception. I have nothing about items in any of my code, so I don't even know where it is coming from.

All help gratefully recieved.