The definition of Form at http://wtforms.readthedocs.io/en/latest/forms.html#wtforms.form.Form.__init__ has meta=None, which should be preserved by subclasses. Since 0.14, Flask-WTF assumes that meta is a dict if present in kwargs. This breaks wtforms-json, which preserve the WTForms default. See also https://github.com/kvesteri/wtforms-json/issues/47.