$ python3 -m pytest -vra
        elif server is not None and server[1] is not None:
            host = server[0]

            # If SERVER_NAME is IPv6, wrap it in [] to match Host header.
            # Check for : because domain or IPv4 can't have that.
            if ":" in host and host[0] != "[":
                host = f"[{host}]"

            host = f"{host}:{server[1]}"
        else:
            host = ""

        if scheme in {"http", "ws"}:
            host = host.removesuffix(":80")
        elif scheme in {"https", "wss"}:
            host = host.removesuffix(":443")

        if not host_is_trusted(host, trusted_hosts):
>           raise SecurityError(f"Host {host!r} is not trusted.")
E           werkzeug.exceptions.SecurityError: 400 Bad Request: Host '' is not trusted.

/usr/lib/python3/site-packages/werkzeug/sansio/utils.py:141: SecurityError
=========================== short test summary info ============================
FAILED tests/test_app.py::test_propagation[True-False-True] - werkzeug.except...
FAILED tests/test_app.py::test_propagation[False-True-True] - werkzeug.except...
FAILED tests/test_app.py::test_propagation[True-True-True] - werkzeug.excepti...