Api platform - invalid IRI instead of validation error

I'm using API platform for my booking application.

Booking has many to 1 relationship with a department.

When I create a new booking by sending a POST request with an empty string as a value for the department I am expecting a validation error but instead, I got 'invalid IRI'

Booking entity

/** * @ApiResource( *      itemOperations={ *          "get"={ *              "normalization_context"={ *                  "groups"={"read-item"} *              } *          }, *          "put"={ *              "denormalization_context"={ *                  "groups"={"put"} *              } *          } *    } * @ORM\Entity(repositoryClass="App\Repository\BookingRepository")class Booking{    /**     * @ORM\ManyToOne(targetEntity="App\Entity\Department", inversedBy="bookings")     * @ORM\JoinColumn(nullable=false)     * @Assert\NotBlank     * @Groups({"read-item", "put"})     */    private $department;}

Department entity

/** * @ApiResource( *     collectionOperations={"get"}, *      itemOperations={"get"} * ) * @ORM\Entity(repositoryClass="App\Repository\DepartmentRepository") */class Department{    /**     * @ORM\OneToMany(targetEntity="App\Entity\Booking", mappedBy="department")     */    private $bookings;}

e.g sample request to create a booking (POST)

{"department": ""}

Error:

{"@context": "/api/contexts/Error","@type": "hydra:Error","hydra:title": "An error occurred","hydra:description": "Invalid IRI \"\".","trace": [        {"namespace": "","short_class": "","class": "","type": "","function": "","file": "/var/www/html/vendor/api-platform/core/src/Serializer/AbstractItemNormalizer.php","line": 418,"args": []        },        {"namespace": "ApiPlatform\\Core\\Serializer","short_class": "AbstractItemNormalizer","class": "ApiPlatform\\Core\\Serializer\\AbstractItemNormalizer","type": "->","function": "denormalizeRelation","file": "/var/www/html/vendor/api-platform/core/src/Serializer/AbstractItemNormalizer.php","line": 685,"args": [                ["string","department"                ],                ["object","ApiPlatform\\Core\\Metadata\\Property\\PropertyMetadata"                ],                ["string","App\\Entity\\Department"                ],                ["string",""                ],                ["string","json"                ],                ["array",                    {"operation_type": ["string","collection"                        ],"collection_operation_name": ["string","post"                        ],"api_allow_update": ["boolean",                            false                        ],"resource_class": ["string","App\\Entity\\Department"                        ],"input": ["null",                            null                        ],"output": ["null",                            null                        ],"request_uri": ["string","/api/bookings"                        ],"uri": ["string","http://localhost/api/bookings"                        ],"skip_null_values": ["boolean",                            true                        ],"api_denormalize": ["boolean",                            true                        ],"cache_key": ["string","db505854694e53aec6831eb427a03028"                        ]                    }                ]            ]        }, }

Please advise if I'm missing something