mocking findOneBy"field" in doctrine2 with phpunit

If I mock the repository method find I got the expected results, but if I method with either findBy, findOneBy, findOneById I've got alway null.

example:

$mock->expects($this->once())
                ->method('getId')
                ->will($this->returnValue(1));
$mockRepository->expects($this->any())
                         ->method('findBy') //if here I use 'find' works for all other cases always null
                         ->will($this->returnValue($mock));

Is there a reson why this happen? is it possible to mock the "magics" method of doctrine2 like findById or findOneById? if it yes how is wrong in my approach?