respond to - Trying to alias old method and create new one in ruby -

i'm using following snippet alias method if method exists:

alias_method old_name, func_name if self.respond_to? func_name 

how can error alias_method': undefined method 'get' class 'sinatra::base' (nameerror) when self.respond_to? func_name returns true?

func_name = :get in snippet

i bet call alias_method inside scope of sinatra::base. should call inside singleton class of sinatra::base, because method get defined class method.