Submit form: error on creating a project on Sib-app

I would like to create a new project but I have an error 500 in the post request.

I have that error message in my Inspector > Network > Response

ValueError at /projects/
Cannot assign "OrderedDict([('name', 'Happy Dev'), ('address', 'une Rue'), ('logo', 'https://avatars.slack-edge.com/2017-07-10/211204058102_d0e79b6b617446e4beb7_512.png'), ('companyRegister', '08698664653760870'), ('firstName', 'Gaëlle'), ('lastName', 'Morin'), ('role', 'someone'), ('email', 'morin.gaelle@gmail.com'), ('phone', '0834567898')])": "Project.customer" must be a "Customer" instance.

And in my Inspector > Network > Headers > Request Payload

{
  "@context": "http://owl.openinitiative.com/oicontext.jsonld",
  "@graph": [
    {
      "@id": "./",
      "http://happy-dev.fr/owl/#businessProvider": "Gaëlle Morin",
      "http://happy-dev.fr/owl/#businessProviderFee": 5,
      "http://happy-dev.fr/owl/#customer": {
        "@id": "_:b18915"
      },
      "http://happy-dev.fr/owl/#description": "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has su",
      "http://www.w3.org/2000/01/rdf-schema#label": "sib-app"
    },
    {
      "@id": "_:b18915",
      "http://happy-dev.fr/owl/#address": "une Rue",
      "http://happy-dev.fr/owl/#companyRegister": "08698664653760870",
      "http://happy-dev.fr/owl/#email": "morin.gaelle@gmail.com",
      "http://happy-dev.fr/owl/#firstName": "Gaëlle",
      "http://happy-dev.fr/owl/#lastName": "Morin",
      "http://happy-dev.fr/owl/#logo": "https://avatars.slack-edge.com/2017-07-10/211204058102_d0e79b6b617446e4beb7_512.png",
      "http://happy-dev.fr/owl/#phone": "0834567898",
      "http://happy-dev.fr/owl/#role": "someone",
      "http://www.w3.org/2000/01/rdf-schema#label": "Happy Dev"
    }
  ]
}