FatalError when flushing after persisting datas - doctrine-orm

Question

I want to persist datas from a new Entity into the database, one of the most basic operation right?
But when I flush, I get the following fatal error:
FatalErrorException in DateTimeType.php line 53:
Error: Call to a member function format() on string
So here's how I want to persist my datas, on Symfony 3.2, PHP 5.6 (sigh) and mariadb 10.3:
$myEntity = new myEntity();
$myEntity->setFoo('a');
$myEntity->setBar('b');
$myEntity->setDateCreation(new \DateTime());
$this->em->persist($myEntity);
$this->em->flush();
My entity:
class myEntity
{
// ...
/**
* #var string
*
* #ORM\Column(name="sFoo", type="string", length=255, nullable=false, unique=true)
*/
private $foo;
/**
* #var string
*
* #ORM\Column(name="sBar", type="string", length=255, nullable=false)
*/
private $bar;
/**
* #var \DateTime
*
* #ORM\Column(name="dDateCreation", type="date")
*/
private $dateCreation;
// Getter and setters are classics, here's the one for the date
public function setDateCreation($dateCreation)
{
$this->dateCreation = $dateCreation;
}
}
I get absolutely no error if I stop the script before flushing. When I dump my entity just before, all the properties are set correctly :
MyEntity {#2301 ▼
-id: null
-foo: "a"
-bar "b
-dateCreation: DateTime {#2204 ▼
+"date": "2020-07-06 12:32:17.000000"
+"timezone_type": 3
+"timezone": "Europe/Paris"
}
}
From the database side : the columns seems to be correctly set: sFoo and sBar are VARCHAR(255) dDateCreation is date.
I eventually changed the date fields type from date to string, with a string as argument like "20200706", just for trying. And... I get the same error, even if there are no DateTime anymore involved in this.
Either I'm completely blind and there is an obvious thing I forgot in the code I've posted, or the reason comes from elsewhere.
I tried the same operation with an other Entity, same result.