Some user cannot login and got 500 return.

[chaintng]

Here is some log,
As you can see, validateCallback show undefined.
It happen to specific users in bitium.
Have someone has any idea how could i debug this problem?

2016-06-01T05:32:29UTC mobilebackend-live[web.1]: [ 'debug', 'SAMLResponse' ]
2016-06-01T05:32:29UTC mobilebackend-live[web.1]: [ 'debug', 'processValidlySignedAssertion' ] '_bff176f63b037c442cdf'
2016-06-01T05:32:29UTC mobilebackend-live[web.1]: [ 'debug', 'validateCallback' ] undefined
2016-06-01T05:32:29UTC mobilebackend-live[web.1]: 160601/053229.578, response, http://0.0.0.0:5000: post /login/callback {} 500 (14ms)