Schema validation issue with a Cornice Service and a Nested Marshmallow Schema

Question

I'm trying to define a Marshmallow Schema with a list of nested fields and use it as a validator for a Cornice Service. The Schema validation works as intended but I got "Not a valid list" error for my associations fields.

My code :

from cornice import Service
from cornice.validators import marshmallow_body_validator

from marshmallow import Schema, fields

# My Cornice Service :

associate = Service(
    name="associate",
    path="/projets/{project_id}/associate",
    description="Associate model to Project",
)

# My Marshmallow Schemas :
class AssociationsSchema(Schema):

    left_id = fields.Int(required=True)
    right_id = fields.Int(required=True)

class GenericAssociationSchema(Schema):

    left_model = fields.Str(required=True)
    right_model = fields.Str(required=True)
    associations = fields.List(fields.Nested(AssociationsSchema))

# POST request
@associate.post(schema=GenericAssociationSchema, validators=(marshmallow_body_validator,))
def associate_post(request):
    return {"success": True}

My pytest tests :

# testapp is a custom pytest fixture
def test_load_schema(testapp):
    json = {
        "left_model": "projet",
        "right_model": "individu",
        "associations": [
            {
                "left_id": 1,
                "right_id": 5,
                "additionnal_fields": {
                    "fonction_id": 3,
                    "role_id": 1
                }
            }
        ]
    }

    schema = GenericAssociationSchema()
    result = schema.load(json)
    assert result == {}
    # PASSED

def test_post_associate_projectindividu_success(testapp):
    response = testapp.post(
        "/projets/1/associate",
        {
            "left_model": "projet",
            "right_model": "individu",
            "associations": [
                {
                    "left_id": 1,
                    "right_id": 5,
                    "additionnal_fields": {
                        "fonction_id": 3,
                        "role_id": 1
                    }
                }
            ]
        },
        status=400,
    )

    assert response.json == {"success": "true"}
    # FAILED : AssertionError: assert {'status': 'error', 'errors': [{'location': 'body', 'name': 'associations', 'description': ['Not a valid list.']}]} == {'success': 'true'}

Without the Nested List of Schema, the service is working fine. Any ideas why I'm getting this error ?