Calling a method of a mock object

I am testing a service with PHPUnit.

I am using setUp() to create mock objects of the dependencies that my service accepts, like this:

 public function setUp()
    {
        $this->fooManagerMock = $this->getFooManagerMock();
        $this->barManagerMock = $this->getBarManagerMock();
        $this->treeManagerMock = $this->getTreeManagerMock();
        $this->loggerMock = $this->getLoggerMock();

     $this->myService = new MyService($this->fooManagerMock, $this->barManagerMock, $this->treeManagerMock, $this->loggerMock);
    }

Bundle\Tests\Service\MyTest::testServiceWithValidData

   /**
     * @dataProvider getServiceValidCaseData
     */
    public function testServiceWithValidData($case)
    {
         $this->assertTrue($this->myService->serve($fooArray));
    }

It turns out that a entity manager that in my serve() function, I am having something like this:

return $this->fooManager->find($fooId) !== null;

in this case $this->fooManager is instance of Mock_FooManager_4038382a and the whole test fails.

Any suggestions are more than welcome.