PHPUnit: Mocking __get() results in "__get() must take exactly 1 argument ..."

I've got a problem with mocking an overloaded __get($index) method. The code for the class to be mocked and the system under test that consumes it is as follows:

<?php
class ToBeMocked
{
    protected $vars = array();

    public function __get($index)
    {
        if (isset($this->vars[$index])) {
            return $this->vars[$index];
        } else {
            return NULL;
        }
    }
}

class SUTclass
{
    protected $mocky;

    public function __construct(ToBeMocked $mocky)
    {
        $this->mocky = $mocky;
    }

    public function getSnack()
    {
        return $this->mocky->snack;
    }
}

Test looks as follows:

<?php    
class GetSnackTest extends PHPUnit_Framework_TestCase
{
    protected $stub;
    protected $sut;

    public function setUp()
    {
       $mock = $this->getMockBuilder('ToBeMocked')
                     ->setMethods(array('__get')
                     ->getMock();

       $sut = new SUTclass($mock);
    }

    /**
     * @test
     */
    public function shouldReturnSnickers()
    {
        $this->mock->expects($this->once())
                   ->method('__get')
                   ->will($this->returnValue('snickers');

        $this->assertEquals('snickers', $this->sut->getSnack());
    }
}

Real code is a little bit more complex, though not much, having "getSnacks()" in its parent class. But this example should suffice.

Problem is I get the following error, when executing the test with PHPUnit:

Fatal error: Method Mock_ToBeMocked_12345672f::__get() must take exactly 1 argument in /usr/share/php/PHPUnit/Framework/MockObject/Generator.php(231)

When I debug I can't even reach the test method. It seems it breaks at setting up the mock object.

Any ideas?