Sort by two date fields in the ORDER BY Expression box.

I have a bunch of News articles that I am listing down a page with a repeater. Some of the news articles have a DocumentPublishFrom date and others do not.

I'd like to order the articles by DocumentPublishFrom, but if there isn't a DocumentPublishFrom, then I'd like to use DocumentCreatedWhen.

I am using the following code in the ORDER BY expression field (clicking on the black arrow) but it seem to not be working properly. Am I doing something wrong?

{% If (DocumentPublishFrom<DocumentCreatedWhen) { "DocumentCreatedWhen DESC" } else {"DocumentPublishFrom DESC"} #%}