Error message with Transmitter Tuning - invalid channel option (219) with 4.5.5 & and 4.5.6 not with 4.6dev

ArduCopter Copter 4.5
1

Hello

I tried transmitter tuning on arducopter 4.5.5 as well as with 4.5.6.
I get always the message „Invalid channel option (219)“. I tried it with different tuning parameters, always error message. Installing 4.6dev it works flawless without any error message.
FC: Holybro Pixhawk 6c mini

BR

Harald