Error: Called id for nil, which would mistakenly be 4

rubyonrails-talk
1

I define a class:

class BuildLogger   @@build_logger=nil

  def self.get_build_logger(file_name=nil)     if @@build_logger == nil ###this line will thrown an error       @@build_logger=Logger.new("build_logger")       @@build_logger.level = DEBUG     end     return @@build_logger   end end

I invoke BuildLogger.get_build_logger in a controller. An error "Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id" is always thrown on the "if" line. I have no idea about this error. does anyone know this problem?