Cannot Make Response Schema

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 "", line 1, in
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:
``` python
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!

Start by reproducing the failure from helpers/check_db_available.py and follow app.py into core/loaders/resource.py, where app.docs.register processes HealthCheckResource. Inspect the HealthCheckResponseSchema used by marshal_with and confirm that the health-check response can register successfully and appear in the generated docs without the AttributeError.

Written by the indexing model from the issue text.