HTTP 522 when using Worker as fallback origin for Cloudflare for SaaS

I'm trying to use a Cloudflare Worker as my fallback origin for Cloudflare for SaaS.

I have the worker registered with a custom domain at fallback.mydomain.com. When I hit it directly, I see the response from the Worker.

I then have fallback.mydomain.com registered as my fallback origin in SSL/TLS > Custom Hostnames.

I have a custom domain registered (different zone, under the same account) and the certificate and hostname status are "Active", but when I hit the custom domain I get an HTTP 522 back.
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