The 1.6.x release of guzzlehttp/psr7 appears to have introduced additional checks and verifications that break tests for Guzzle Services. This persists even with version 1.6.1 of guzzlehttp/psr7 which supposedly had rolled-back several type assertions to defer them until 2.0.0.

As a workaround, tests pass if guzzlehttp/psr7 is downgraded to 1.5.2 with composer require guzzlehttp/psr7:1.5.2.

Test failures are attached.
failures.log

This affects the master branch at 9e3abf2.