Impossible to trigger the camera using the PWM signal from an AUX port on CUAV V5nano.

I’m setting up a copter running PX4 v1.10 with CUAV V5nano (FMU v5) as hardware. Everything works fine until I try to set up the camera trigger, I need to set up the camera trigger with a PWM signal coming from an AUX pin. if I set the AUX port 5 or 6 as the port for the trigger, the FC will emit the sound of Program PX4IO Fail after the boot up, indicating AUX Mixer not found. Then the camera trigger will work faultlessly but when arming the copter the motors won’t spin. The only way to prevent the copter from this boot-up error is to select port 5 and 6 simultaneously for camera trigger. Then the copter will boot up normally and when it is armed the motors will spin as they should. But the camera trigger will not work on either port 5 or 6.

What could be happening here? Any way to solve this problem?