Chitika not showing alternate ads when using fluid height

I’m trying to use a Chitika ad unit with an alternate ad (so all those non-US people still get to see some ads :wink: ). The problem is as follows: when I use a Chitika ad with fluid height (generated in their “Get a List Unit” section), the alternate ads do not display, despite alternate ad URL being specified! Chitika ad unit just collapses when it doesn’t have an ad to show.

One way around this problem is to generate the code using their “Get Started > Get Code” section, which specifies a fixed height (e.g. 550x250). The issue with that is, if I use a 468x60 alternate ad, all that empty space below it looks quite ugly (and I don’t really want to use a 468x60 Chitika ad, as I expect it get a lot less clicks).

How come alternate ads do not display when fluid height is used? I mean, Chitika themselves give the option of an alternate ad URL - why would they do that if it isn’t working?

Maybe the alternate ad needs to have some sort of code in it that prevents Chitika ad from collapsing? It’s an iframe and it already has width and height specified in it.