I have a basic form that has two fields: one a normal text field, and one an entity field defined like this:

$builder->add('division', 'entity', [
    'class' => 'MyApp:Division',
    'property' => 'division',
    'required' => false
]);

This renders the field properly on the screen: it has an empty <option> tag and then the rest of the MyApp:Division entities loaded.

If I submit the form through a GET request and handle it like this:

if ($request->query->has($searchForm->getName())) {
    $searchForm->submit(
        $request->query->get($searchForm->getName())
    );
}

I get a Doctrine exception thrown saying it can not execute a query with an empty IN() clause (truncated for clarity):

An exception occurred while executing 'SELECT ... WHERE d0_.id IN (?)' with params [""]: SQLSTATE[22P02]: Invalid text representation: 7 ERROR: invalid input syntax for integer: "" /var/apps/my-app/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/AbstractPostgreSQLDriver.php 91

Curiously, if I set the choices key in the form options, an empty submission works:

// Assume $this->divisions was passed in through the constructor.
$builder->add('division', 'entity', [
    'class' => 'MyApp:Division',
    'choices' => $this->divisions
    'property' => 'division',
    'required' => false
]);

I've narrowed down the offending code to the Symfony\Bridge\Doctrine\Form\ChoiceList\DoctrineChoiceLoader class on line 143 in the loadChoicesForValues() method.

The $values array looks like this when the choices key is absent and an empty field is submitted:

array (
  0 => ''
)

Which causes the IN() query to fail.