I was playing with a

http://www.mobisoftinfotech.com/blog/iphone/ios-tutorial-custom-speedometer-control/

It turned out I added a control ‘test’, linked it to an IBOutlet and then removed the control. Since the link to IBOutlet is sTILl in the IB file, hence the error.

By right mouse click on the ‘Owner’ and remove the invalid link, it fixes the problem.