PHP class not found error ONLY happens on CircleCI

I'm not seeing any errors when I run or test my code locally. My phpunit test suite passes:

Time: 11.69 seconds, Memory: 43.50Mb

OK, but incomplete, skipped, or risky tests!
Tests: 149, Assertions: 281, Incomplete: 13.

But when I push the exact same code and CircleCI runs the exact same test command, it fails with a fatal error:

.PHP Fatal error: Class 'App\Domain\API\Request\Soap\Json\AbstractJson' not found in /home/ubuntu/TransitScreen/src/Domain/Api/Request/Soap/Json/NJTransit.php on line 10 php ./vendor/bin/phpunit --no-coverage --testsuite=unit returned exit code 255

At first I thought maybe CircleCi was running an old version of PHP without namespaces or something but even when I set it to use PHP v5.6.5 the same error happens. I even tried adding a database dump into the test sequence.

It's even stranger to me that it is able to run several dozen tests before the error happens. So I'm pretty sure that it's not something grossly misconfigured on CircleCI.

Any suggestions on why the same code would behave so differently in the two environments?