Hi! I raised this in the #metabrainz IRC too some time ago and this seems like a bug with CAA. CAA returns an inconsistent response for the ID parameter for some queries. For example:

1. https://coverartarchive.org/release/76df3287-6cda-33eb-8e9a-044b5e15ffdd?fmt=json
2. https://coverartarchive.org/release/18d4e9b4-9247-4b44-914a-8ddec3502103?fmt=json

The value of ID in the 1st URL is represented as an integer (not surrounded in quotes). However, in the 2nd URL, it is represented as a string (surrounded in quotes).

@atj also created this gist for better demonstration.