Gruff Bar Chart Stop Showing up After Rails 3.2.8+ Upgrade

I am using rails 3.2.8 with the gruff gem 0.3.7 and it works great. Once I upgrade to a higher version of rails 3 my gruff charts stop showing up and I get the following errors in my logs: TypeError (no implicit conversion of nil into String): app/controllers/report_controller.rb:64:in delete' app/controllers/report_controller.rb:64:inensure in generate_chart' app/controllers/report_controller.rb:64:in `generate_chart' ....

Any advice would be great.