Why doesn't my if-then statement in Scratch ever trigger?

I have been using Scratch for a few months, but am suddenly completely and inexplicably stuck. I can't get a simple if-then condition to trigger. After stripping down to the bare essentials, I have this:

the variable d successfully changes when I press space, but never triggers the if-then, even when d=5, as confirmed by the display. What am I missing?