Notice: Array to string conversion with checkboxes

I have a form where I add missions. One of the elements inside the forms is a multiple checkbox (those who need to be ticked) The problem is that I never used Symfony's checkbox before so I'm a little bit lost. The form displays properly but when I try to submit the form, I get the following error:

Notice: Array to string conversion

Here is my entity (at least the concerned part):

//...

/**
* @ORM\Column(name="typeContrat", type="string")
*/
private $typeContrat;

//....

and here is how I added it into the form builder:

    $contrats = array(
        'stage' => 'stage',
        'stage alterné' => 'stage alterné',
        'contrat de professionnalisation' => 'contrat de professionnalisation',
        'contrat d\'apprentissage' => 'contrat d\'apprentissage'
    );

    $builder
        //...
        ->add('typeContrat', ChoiceType::class, array(
            'multiple' => true,
            'expanded' => true,
            'choices' => $contrats,
        ));

I guess the error comes from the fact that I said "expect a string" but I give him an array instead. So how can do to save whatever the user ticks into my database ?

Thank you in advance