Generating URL with Symfony Router component in PHPUnit test

I am attempting to write a test in PHPUnit for a simple method which returns a URL based on a Symfony route for a subdomain. I'm not sure if my test has proper access to the Symfony Router component and that is why it is falling over, but I can't quite seem to get it to pass at the moment.

This is the error I am getting:

There was 1 error:

1) CRMPiccoBundle\Tests\Services\MailerHelperTest::testReturnsBillingURL with data set "crmpicco.co.uk" (CRMPiccoBundle\Entity\Course Object (...), 'http://ift.tt/2d4xfOp') TypeError: Return value of CRMPiccoBundle\Services\MailerHelper::getBillingUrlForSubdomain() must be of the type string, null returned

This is my test and data provider:

/**
 * @dataProvider dataProvider
 */
public function testReturnsBillingURL(Course $course, string $expectedUrl)
{
    $mailerHelper = new MailerHelper($this->getRouterMock());
    $this->assertEquals($expectedUrl, $mailerHelper->getBillingUrlForSubdomain($course, true));
}

public function dataProvider()
{
    return [
        'crmpicco.co.uk' => [
            'site' => (new Course())
                ->setSubdomain('crmpicco'),
            'expectedUrl' => 'http://ift.tt/2d4xfOp',
        ],
    ];
}

This is my getRouterMock method:

/**
 * @return \Symfony\Bundle\FrameworkBundle\Routing\Router
 */
protected function getRouterMock()
{
    return $this->getMockBuilder('\Symfony\Bundle\FrameworkBundle\Routing\Router')
        ->disableOriginalConstructor()
        ->setMethods(['generate', 'supports', 'exists'])
        ->getMockForAbstractClass();
}

This is the method I am testing:

/**
 * @param Course $course
 * @param bool $absoluteUrl
 *
 * @return string
 */
public function getBillingUrlForSubdomain(Course $course, bool $absoluteUrl = true) : string
{
    return $this->router->generate('crmpiccobundle_portal_billing', [
        'subdomain' => $site->getSubdomain(),
    ], $absoluteUrl);
}