After the recent update of guzzlehttp/promises to v2 (https://github.com/guzzle/promises) with breaking changes this package is broken by it:

PHP Fatal error: Uncaught Error: Call to undefined function GuzzleHttp\Promise\inspect_all() in /vendor/kevinrob/guzzle-cache-middleware/src/CacheMiddleware.php:113

I'd guess a fix would be to explicitly depend on guzzlehttp/promises, either v1 or v2, or both somehow.