ONION_CLIENT_AUTH_ADD persistence error unhelpfully vague

I tried to add an integ test for persisting hidden service credentials to disk (calling ONION_CLIENT_AUTH_ADD with a "Permanent" flag) but the error response I received from tor is unhelpfully vague...

Unable to store creds for "yvhz3ofkv7gwf5hpzqvhonpr3gbax2cc7dee3xcnt7dmtlx2gu7vyvid"

It's possible that there's an issue on my end, or also possible that the feature doesn't work. Unfortunately this response is too nebulous for me to troubleshoot.

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