Cannot Make Response Schema

[sharokh23]

I am trying to define a simple resource with a sample response schema so as to display in the docs. I only have two attributes in the class and every time I am trying to run it I get:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/tenacity/__init__.py", line 333, in wrapped_f
    return self(f, *args, **kw)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/tenacity/__init__.py", line 423, in __call__
    do = self.iter(retry_state=retry_state)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/tenacity/__init__.py", line 360, in iter
    return fut.result()
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
    raise self._exception
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/tenacity/__init__.py", line 426, in __call__
    result = fn(*args, **kwargs)
  File "/home/app/amp/helpers/check_db_available.py", line 14, in main
    app = create_app()
  File "/home/app/amp/app.py", line 92, in create_app
    load_resources(app)
  File "/home/app/amp/core/loaders/resource.py", line 19, in load_resources
    app.docs.register(r, endpoint=r.endpoint)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/flask_apispec/extension.py", line 127, in register
    self._defer(self._register, target, endpoint, blueprint,
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/flask_apispec/extension.py", line 71, in _defer
    bound()
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/flask_apispec/extension.py", line 157, in _register
    self.spec.path(**path)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/core.py", line 534, in path
    plugin.operation_helper(path=path, operations=operations, **kwargs)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/__init__.py", line 224, in operation_helper
    self.resolver.resolve_operations(operations)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/schema_resolver.py", line 35, in resolve_operations
    self.resolve_response(response)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/schema_resolver.py", line 183, in resolve_response
    self.resolve_schema(response)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/schema_resolver.py", line 225, in resolve_schema
    data["schema"] = self.resolve_schema_dict(data["schema"])
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/schema_resolver.py", line 295, in resolve_schema_dict
    return self.converter.resolve_nested_schema(schema)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/openapi.py", line 134, in resolve_nested_schema
    self.spec.components.schema(name, schema=schema)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/core.py", line 171, in schema
    ret.update(plugin.schema_helper(component_id, ret, **kwargs) or {})
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/__init__.py", line 182, in schema_helper
    json_schema = self.converter.schema2jsonschema(schema_instance)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/openapi.py", line 253, in schema2jsonschema
    jsonschema = self.fields2jsonschema(fields, partial=partial)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/openapi.py", line 278, in fields2jsonschema
    prop = self.field2property(field_obj)
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/field_converter.py", line 172, in field2property
    ret.update(attr_func(field, ret=ret))
  File "/home/app/cache/virtualenvs/amp-_geWzZZ1-py3.8/lib/python3.8/site-packages/apispec/ext/marshmallow/field_converter.py", line 223, in field2default
    default = field.load_default
AttributeError: 'String' object has no attribute 'load_default'

I am defining it according to the documentation:

from amp.core.models.base import APIResource
from flask_apispec import doc, marshal_with
from marshmallow import Schema, fields
from amp.core.extensions.db import db

__all__ = ('HealthCheckResource',)

class HealthCheckResponseSchema(Schema):
    status = fields.String(default="ok")
    db = fields.String(default="ok")

class HealthCheckResource(APIResource):
    url = "/api/healthcheck"
    endpoint = "api.healthcheck"

    @doc(
        summary="General health check",
        description="My First GET Awesome API.",
        tags=['Health Check'],
    )
    @marshal_with(
        schema=HealthCheckResponseSchema,
        description="Successful operation",
        code=200)
    def get(self):
        db_status = "ok"
        try:
            db.engine.execute("SELECT 1")
        except Exception as e:
            db_status = db_status = "degraded"
        statuses = [
            db_status
        ]

        if any([s != "ok" for s in statuses]):
            return {
                "status": "degraded",
                "db": db_status
            }
        else:
            return {
                "status": "ok",
                "db": db_status,
            }

Any help is much appreciated!