phpunit ock returns null while original method returns string

I have this file structure:

- module
-- phpunit.xml
-- blaat.php
-- tests
--- blaatTest.php

contents of blaat.php

class Blaat
{
    public function doSomething()
    {
        return 'my return value';
    }
}

contents of tests/blaatTest.php

use PHPUnit\Framework\TestCase;

require_once './blaat.php';

class blaatTest extends TestCase
{ 
    public function testCanBeCreatedFromValidEmailAddress(): void
    {
        $stub = $this->createMock(Blaat::class);

        $this->assertEquals('foo', $stub->doSomething());
    }
}

contents of phpunit.xml

<?xml version="1.0" encoding="UTF-8"?>
<phpunit
        bootstrap="./tests/bootstrap.php">
    <testsuites>
        <testsuite name="unit">
            <directory>tests</directory>
        </testsuite>
    </testsuites>
    <filter>
        <blacklist>
            <directory suffix=".php">vendor</directory>
        </blacklist>
    </filter>
</phpunit>

When I run phpunit in my terminal (when I'm in the modules folder) I get this:

PHPUnit 6.0.8 by Sebastian Bergmann and contributors.

F                                                     1 / 1 (100%)

Time: 427 ms, Memory: 22.00MB

There was 1 failure:

1) blaatTest::testCanBeCreatedFromValidEmailAddress
Failed asserting that null matches expected 'foo'.

/path/to/module/tests/blaatTest.php:19

FAILURES!
Tests: 1, Assertions: 1, Failures: 1.

How is this possible? My method will always return a string, yet phpunit says it gets a null value as a return. What am I doing wrong here?