Digital change detection not working on PXIe-6535 / .NET DAQmx

I'm trying to block program excecution until PXI1Slot6/Port1/Line1 goes high, but the Task.DigitalChangeDetection event never fires.

ManualResetEventSlim waitForTrigger = new ManualResetEventSlim();
        using (Task task = new Task())
        {
          task.DIChannels.CreateChannel("PXI1Slot6/Port1/Line1", "", ChannelLineGrouping.OneChannelForAllLines);
          task.Start();
          task.DigitalChangeDetection += (e, o) =>
           {
             waitForTrigger.Set();
           };
          waitForTrigger.Wait();
        }

I can clearly see Port1/Line1 going high and low on the scope, but the DigitalChangeDetection event never fires. According to the documentation on the DigitalChangeEvent :

Occurs when a digital change is detected on any of the digital lines used in the task.

 So why doesn't Port1/Line1 going high low fire the DigitalChangeDetection event?