Adding WatchKit cause Linker error - entry point (_main) undermine for arch arm64

I get this linker error for the WatchKit Extension target after adding watch kit to an existing iOS project. I've not really any ideas where to start in the hunt for the solution to this problem, and can't see any similar threads like it in the forums. My iOS app is a typical ObjC app running on iPhone6+ (for debug). The target architecture is "standard architectures (armv7, arm64) Any ideas?? I am at a loss. Thanks Rob.