How do I skip Brakeman checks?

I run a brakeman check on my Rails app (Brakeman 3.4.1, Rails 3.2.21). There is a config file brakeman.yml which contains the following block of code.

---
:skip_checks:
# Skipping this check because of bug in rails 3.2.13
# http://blog.bugsnag.com/2013/03/20/rails-3-2-13-performance-regressions-major-bugs/
- CheckSymbolDoS

I have verified that removing CheckSymbolDoS causes Brakeman to err. However, when I add new checks to the config file - such as CheckMassAssignment - Brakeman still performs the MassAssignment check. Is there something obvious I'm missing...?