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

Rails
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?