Debug on Apple Watch: Error Launching 'xxx WatchKit Extension' - Couldn’t communicate with a helper application

I am trying to debug on my apple watch (real device, not simulator). My main project runs on my iPhone fine, and the corresponding watch app shows up on my watch. I want to attach the debugger to my watch app, so in Xcode I choose WatchKit App scheme, then choose my iPhone. I hit the run button, and an error window pops up, with title Error Launching 'xxx WatchKit Extension' and description Couldn’t communicate with a helper application.

Both my main app and watch app run fine if I build and run by choosing the main app scheme, however I wish to attach the debugger to my watch app scheme. Does anyone know what might be causing this error? Thanks in advance.