I think you do need the `@`

app/finders/user_finder.rb

@@ -53,6 +53,6 @@ class UserFinder
   end
 
   def input_is_id?
-    username_or_id.is_a?(Numeric) || username_or_id =~ /^\d+$/
+    @username_or_id.is_a?(Numeric) || @username_or_id =~ /^\d+$/
   end
 end