Custom subclass returns Problem str not json

[dineshbvadhia]

Using the custom subclass example at https://github.com/vapor-ware/fastapi-rfc7807/blob/master/examples/basic.py, I have a similar construct below. However, I don't understand why it returns a str and not json. The fastapi-rfc7807 code processing the custom subclass is at https://github.com/vapor-ware/fastapi-rfc7807/blob/46dd14254d4aa09a6180adfb563a018b710ba0a1/fastapi_rfc7807/middleware.py#L110

My code is:

from fastapi_rfc7807.middleware import Problem

class TableNotFoundError(Problem):
    """ """
    headers = {
        'WWW-Authenticate': 'Bearer',
    }

    def __init__(self, msg: str) -> None:
        super(TableNotFoundError, self).__init__(
            status=500,
            detail=msg,
        )

try:
    do_something(name)
except:
    raise TableNotFoundError("Unknown table: %r" % name)

The response is:

TableNotFoundError: Problem:<{'type': 'about:blank', 'title': 'Internal Server Error', 'status': 500, 'detail': "Unknown table: 'hippo'"}>