Stripe - can't get this to work

Wappler 6.8.0 - following the embedded checkout docs and I get the following error in the console: fetchClientSecret failed with error "The fetchClientSecret function should always resolve with a client secret as a string. The function that was provided resolved with a value type of undefined.

I have used the embedded checkout template server side and added my test secret. I have added the stripe embedded checkout app connect with my publishable key to my page. The session Url is set to my server connect API. When I open the page I get a message saying "Something went wrong. Please try again or contact the merchant" and the above error message is in the console.
I have been working at this for several days and suspect it might be a bug.

The server side:

A blank page with the Stripe Embedded Checkout App Connect linked to server connect. It is faint but you can just make out the error message on the page:

This is the blank page with the Stripe EMbedded Checkout component so you can see the error: Untitled Document

Ok, I have made progress. The error is misleading. It seems it will throw this error if you don't have line items i.e. the secret does not get fetched if there are no line items.