Node Inspector as of v0.12.8 seems broken when paired with Node v6.5.0 (at least on OS X El Capitan). When firing up node-debug, the browser is launched in order to debug the application as per usual, but the debugger fails quickly due to an exception caused by InjectorClient.prototype._findNMInScope not finding any 'NativeModule' property. See Node Inspector issue #905 for reference.
How can Node Inspector be made to work with Node v6.5.0?
Looks like there's native support for inspection in Node from v6.5.0, so Node Inspector might no longer be required. It can be invoked like so: node --inspect --debug-brk my-app.js. I've tried it, and it seems to basically work although I've yet to figure out how to actually use it.