Problems running built-in server under PHP 7.1.21 on Fedora 27

[CyberiaResurrection]

I'm setting up a mock under the above PHP version, using PHPUnit 6.5.21, and have run into problems simply trying to get an empty test to run. I haven't hooked up any specific framework, beyond the bits that http-mock itself pulls in.

Here's the guts of my base test class:

<?php
namespace Tests;

use PHPUnit\Framework\TestCase as BaseTestCase;
use InterNations\Component\HttpMock\PHPUnit\HttpMockTrait;

abstract class TestCase extends BaseTestCase

    use HttpMockTrait;	
    public static function setUpBeforeClass()
    {
        static::setUpHttpMockBeforeClass(null, 'localhost');
    }

    public static function tearDownAfterClass()
    {
        static::tearDownHttpMockAfterClass();
    }
    public function setUp()
    {
        $this->setUpHttpMock();
        parent::setUp();
        $this->http->mock
            ->when()
                ->methodIs('POST')
            ->then()
                ->body('')
            ->end();
        $this->http->setUp();
    }
    public function tearDown()
    {
        $this->tearDownHttpMock();
        parent::tearDown();
    }
}

And the test in question

class baseTest extends TestCase {
    public function testSimpleRequest()
    {
        $request = $this->http->requests->latest();
    }
}

Test result:

1) ...\Tests\baseTest::testSimpleRequest
UnexpectedValueException: Expected status code 200 from "/_request/last", got 404

.../vendor/internations/http-mock/src/RequestCollectionFacade.php:158
.../vendor/internations/http-mock/src/RequestCollectionFacade.php:141
.../vendor/internations/http-mock/src/RequestCollectionFacade.php:27
.../tests/baseTest.php:18

I'm not sure what else I can trim out. Have I managed to do something dumb like leave out a "start the mock server" step? From a quick read through the getting started guide, it doesn't seem like that.