flask 提示 TypeError: Must provide a session-like object as csrf context

已经设置了csrf

from flask.ext.wtf import Form
from wtforms.fields import TextField, BooleanField,StringField,SubmitField
from wtforms.validators import Required
from wtforms.csrf.session import SessionCSRF
from datetime import timedelta

class MyBaseForm(Form):
class Meta:
csrf = True
csrf_class = SessionCSRF
csrf_secret = b'EPj00jpfj8Gx1SjnyLxwBBSQfnQ9DJYe0Ym'
csrf_time_limit = timedelta(minutes=20)

class SignupForm(MyBaseForm):

name = StringField('What is your name?', validators=[Required()])
submit = SubmitField("Create account")