Correction to my previous response…
The way I registered the topic schema is actually this:
curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" --data '{ "schema": "[ { \"name\": \"io.confluent.examples.avro.Customer\", \"subject\": \"customer\", \"version\": 1 }, { \"name\": \"io.confluent.examples.avro.Product\", \"subject\": \"product\", \"version\": 1 }]" }' http://localhost:8081/subjects/all-types-value/versions
Same error as before.