Hi! I've been trying to do Custom White balance with External flash but keep getting a "Failed" info from the camera.

This is how I did it...I set my camera to M mode, settings of -2EV and aperture of f/4-f/1.8. pointed the flash at 0 and 90 degrees. Used a white balance cap. Pointed the camera on a lit room, white wall and white ceiling. Then when I fired the camera and the flash, the camera gave me info that CWB failed...

Any tips on how to do custom white balance with external flash? thanks!