Can't set GADRequest.Request.TestDevices

The component was working fine, but hen I started to get "no ad to show" errors and it's not working anymore. i'm suspecting that maybe my devices have been locked by adMob (not sure if this is the problem). I'm setting the TestDevices property like this: GADRequest.Request.TestDevices = new string[2] { "ID1", "ID2" };

But after this line of code GADRequest.Request.TestDevices is still null. Why is that?