diff --git a/app/models/derived_variables/sales_log_variables.rb b/app/models/derived_variables/sales_log_variables.rb
index d9af88e65..3d837a6bf 100644
--- a/app/models/derived_variables/sales_log_variables.rb
+++ b/app/models/derived_variables/sales_log_variables.rb
@@ -168,7 +168,7 @@ private
     (start_index..6).each do |idx|
       if age_under_16?(idx)
         self["ecstat#{idx}"] = 9
-      elsif self["ecstat#{idx}"] == 9 && self["age#{idx}_known"] == 0
+      elsif self["ecstat#{idx}"] == 9 && (self["age#{idx}_known"]).zero?
         self["ecstat#{idx}"] = nil
       end
     end