As of December 17, 2018, we have made a change to how a subscription is displayed in the XML API response.
Previously, dates and integers that had a null value still displayed its
<trial-started-at type=\"datetime\" nil=\"true\"/> <next_product_id type="integer" nil="true"/>
Now, for null fields, the type is no longer returned:
<trial-started-at nil=\"true\"/> <next_product_id nil="true"/>