I'm currently adding the datadog agent to an app which is truncating its database each time before any specs are run. Although tracing is supposed to be disabled in the test environment the gem still tries to trace the truncation. So trying to run the specs results in the following error:

NoMethodError: undefined method `delete' for nil:NilClass
  /home/redacted/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/ddtrace-0.18.2/lib/ddtrace/contrib/mongodb/subscribers.rb:53:in `failed'

It turns out that this very line was changed in the most recent version (0.18.2), and downgrading to version 0.18.1 makes the issue go away.