This is a bug in v2.71. What happens is that a new event named OnWindowsFocus is raised when the debug dialog appears and clears the Sender keyword. It will be fixed in the next version.
Ah nice. I was trying to debug a generic button press event and getting exactly the same thing. Which was very confusing, as I was sure it had worked a few weeks ago in another app, but debugging it simply would not work.
Do you email current users when a new version is available?