Bug Application for Symfony ObjectMapper Step to reproduce the bug Clone this repository run composer install run symfony serve Make a POST request with data : curl -X POST http://localhost:8000 -H 'Accept: application/json' -H 'Content-Type: application/json' -d '{"data": [{"value": "my_value"}]}' See that you have an error Fixing Rename services.yaml into services.back.yaml Rename services.fixed.yaml into services.yaml Change new MapCollection() into MapCollection::class in DataDto.php Run the same request as previously See that it works fine Check you have pass thought setter value as value in final object of response