EasyAdminBundle: Validation not working on CKEditorType

In my admin panel created with EasyAdminBundle, my form validations only work with fields that do not have the CKEditorType. Some fields need to be edited so I implemented a WYSIWYG with FOSCKEditorBundle.

Snippet from field concerned:

- { property: 'content', type: 'FOS\CKEditorBundle\Form\Type\CKEditorType'} 

When I submit the form with an empty 'content' field, I get an InvalidArgumentException with the error: Expected argument of type "string", "NULL" given. instead of a validation error like Please fill in this field.

Snippet from field concerned without CKEditor:

- { property: 'content' } 

=> validation works perfectly.

My entity field:

    /**
     * @ORM\Column(type="text")
     * @Assert\NotBlank
     * @Assert\NotNull
     */
    private $content;

The Symfony profiler shows that this field indeed has a required attribute.

How can enable the validations with the CKEditor field type?