SAE本地调试调用set_secure_cookie是报错

index.wsgi代码如下：
import sae
import tornado.wsgi

from blog import urls as blogurls
from admin import urls as adminurls

settings = {
'debug': True,
}

app = tornado.wsgi.WSGIApplication(blogurls + adminurls, **settings)

application = sae.create_wsgi_app(app)

通过 URL 被调用的方法如下：
class GetCaptcha(BaseHandler):
def get(self):
text = generate_random(4)
# self.set_secure_cookie("captcha", text)
self.set_cookie("captcha", text)

strIO = Recaptcha(text)

# ,mimetype='image/png'
self.set_header("Content-Type", "image/png")
self.write(strIO.read())
return

使用set_cookie没有问题，但是当使用 set_secure_cookie 时报错如下：
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/tornado-2.1.1-py2.7.egg/tornado/web.py", line 954, in _execute
getattr(self, self.request.method.lower())(*args, **kwargs)
File "/home/dreambt/PycharmProjects/cms4p/5/admin.py", line 485, in get
self.set_secure_cookie("captcha", text)
File "/usr/local/lib/python2.7/dist-packages/tornado-2.1.1-py2.7.egg/tornado/web.py", line 388, in set_secure_cookie
self.set_cookie(name, self.create_signed_value(name, value),
File "/usr/local/lib/python2.7/dist-packages/tornado-2.1.1-py2.7.egg/tornado/web.py", line 398, in create_signed_value
self.require_setting("cookie_secret", "secure cookies")
File "/usr/local/lib/python2.7/dist-packages/tornado-2.1.1-py2.7.egg/tornado/web.py", line 910, in require_setting
"application to use %s" % (name, feature))
Exception: You must define the 'cookie_secret' setting in your application to use secure cookies

setting.py中已经设置了：
COOKIE_SECRET = 'L8LwECiNRxq2N0N2eGxx9MZlrpmuMEimlydNX/vt1LM='