AttributeError: 'StorageProviderSettings' object has no attribute 'port'

[t-silvers]

a snippet like this:

envvars:
    'SNAKEMAKE_STORAGE_WEBDAV_USERNAME',
    'SNAKEMAKE_STORAGE_WEBDAV_PASSWORD',

storage:
    provider='webdav',
    host='https://<remaining WebDAV URL>',

rule:
    input:
        storage.webdav('dav://<path to file>')
    ...

is raising an AttributeError:

WorkflowError:
Failed to check existence of
dav://<path to file>
AttributeError: 'StorageProviderSettings' object has no attribute 'port'

any idea what's happening? is there a usage error I'm overlooking?