Getting 'shib_check_user threw an exception: basic_string::_S_construct null not valid

[Paul Wilt]

Generated when certain people try to login from
https://auth.athensams.net/oahf/...

2014-09-26 17:25:03 INFO Shibboleth.Config : Shibboleth SP Version 2.5.3
2014-09-26 17:25:03 INFO Shibboleth.Config : Library versions: log4shib
1.0.8, Xerces-C 3.1.1, XMLTooling-C 1.5.3, Shibboleth 1.5.3

I looked online and saw very little

Wondering how I can go about debugging this error.

Thanks,
Paul
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