There's a bug in the current version of net-ldap that causes this exception if you're trying to use SASL authentication. The fix was just merged (ruby-ldap/ruby-net-ldap#281), but the maintainers are waiting to merge a few more PRs before releasing a new version.
I'm opening this issue for anyone else who might stumble across this error.
To the maintainer(s) of omniauth-ldap, once net-ldap gets a version bump, can we point the gemspec at the new version?