diff --git a/app/models/custom_field.rb b/app/models/custom_field.rb
index be9cf550f..e01402af5 100644
--- a/app/models/custom_field.rb
+++ b/app/models/custom_field.rb
@@ -263,7 +263,7 @@ class CustomField < ActiveRecord::Base
   end
 
   def <=>(field)
-    return nil unless field.is_a?(CustomField)
+    return nil unless field.is_a?(self.class)
 
     position <=> field.position
   end