undefined method `fa_icon` for #<#Class:...

If you're trying to use the Font-Awesome gem in your Rails engine, you may run into this "undefined method `fa_icon for #<#Class:.." error. Fortunately, there is a pretty simple fix.

Add

 include FontAwesome::Rails::IconHelper  

to

 engine_name/app/helpers/engine_name/application_helper.rb  

and the error should go away.

Thanks to Github for the solution.