## The problem
Calling the mandate `revoke` method in mollie-api-php 2.12.1 failed with `Non-existent query parameter "testmode" for this API call`. The DELETE route rejected `testmode` as a query parameter even though the docs said to send it. A maintainer confirmed it was a bug in the revoke method: the parameter was appended to the URL instead of being handled for the DELETE request. The maintainer's suggested workaround was to use `$customer->revokeMandate($mandate->id)` instead, which the reporter confirmed worked. Mollie then fixed it on their side by accepting testmode on those URLs.
## The fix
The mandate `revoke` method put `testmode` in the query string, but the DELETE endpoint rejected it there. Fix: call `$customer->revokeMandate($mandate->id)` instead of the mandate resource's `revoke` method. This was the maintainer's suggested workaround and the reporter confirmed it worked. Mollie later fixed the API to accept testmode on these URLs, so newer API behavior handles it too.