Tricopter servo on auxiliary output

I’m trying to get the servo for the yaw mechanism of a tricopter on one of the auxiliary outputs from the pixhawk.

In …/ardupilot/libraries/AP_Motors.AP_MotorsTri.h on line 14 there is a line:

This means it’s on output 7 of the main PWM-outputs of the pixhawk. I can change it to CH_1, CH_2, … CH_8 and it changes the output between one of the main PWM-outputs of the pixhawk. If I change it to CH_9 it uses the first auxiliary PWM-output. However, if I set it to CH_10, CH_11, CH_12, CH_13 or CH_14. It won’t even compile. Anybody knows a the cause for this?

It seems this line was the problem:

hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_CH_TRI_YAW]), _rc_yaw.radio_trim);

You can change AP_MOTORS_CH_TRI_YAW to CH_1 upto CH_9. But from CH_10 and up it will give you compilation errors. I have no idea why.