Sorry, I seem to have given too little information here. The current test cases for this functionality are located in [CompoundFormTest:85++](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Tests/CompoundFormTest.php#L85), so we should also add this test case there. For example:

```php
public function testSubmitForwardsNullIfNotClearMissingButValueIsExplicitlyNull()
{
    $child = $this->getMockForm('firstName');

    $this->form->add($child);

    $child->expects($this->once())
        ->method('submit')
        ->with($this->equalTo(null));

    $this->form->submit(array('firstName' => null), false);
}
```