node-inspector: how to break on watch variables?

I'm writing a simple command-line Node.js application where GLOBAL.thingies is ending up undefined for me mysteriously.

I used node-inspector, and I can set breakpoints at any given line, and I have "watch expressions" for that variable, like I do in any debugger. However, I can't find when that expression is becoming undefined. How can I set a breakpoint whenever a variable changes?

Node-inspector version 0.9.2.

Node version 0.12.2

Chrome version 41