Type conversion error from LifecycleEventArgs - symfony-3.3

Question

I have no idea for this. I'm a beginner of symfony3.
code
/**
* Set createdAt
* #ORM\PrePersist()
* #param \DateTime $createdAt
*
* #return Book
*/
public function setCreatedAt($createdAt)
{
$this->createdAt = $createdAt;
return $this;
}
/**
* Get createdAt
*
* #return \DateTime
*/
public function getCreatedAt()
{
return $this->createdAt;
}
/**
* Set updateAt
*
* #param \DateTime $updateAt
*
* #return Book
*/
public function setUpdateAt($updateAt)
{
$this->updateAt = $updateAt;
return $this;
}
/**
* Get updateAt
*
* #return \DateTime
*/
public function getUpdateAt()
{
return $this->updateAt;
}
/**
* #ORM\PrePersist()
*/
public function PrePersist() {
if ($this->getCreatedAt() == null) {
$this->setCreatedAt(new \DateTime('now'));
}
$this->setUpdateAt(new \DateTime('now'));
}
/**
* #ORM\PreUpdate()
*/
public function PreUpdate() {
$this->setUpdateAt(new \DateTime('now'));
}
it gives an error report as below :
Error
Could not convert PHP value of type 'Doctrine\ORM\Event\LifecycleEventArgs' to type 'datetime'. Expected one of the following types: null, DateTime
Help me to figure it out.