SEN0657 Zero wind/rain

I am using the SEN0657 Ultrasonic 7-in-1 RS485 weather station. Reading values seems to work fine. 

My unit is currently inside and dry (so no wind or rain). I get the following values:

Wind Speed:     0.1
Wind Direction: 180
Rainfall:       0.2

I am trying to zero the wind/rain as described in the documentation. To zero rainfall I send:

0x1 0x6 0x60 0x2 0x0 0x5 0xf6 0x9
 

I then get the same string back as a response, which I guess means it was accepted:

0x1 0x6 0x60 0x2 0x0 0x5 0xf6 0x9
 

…but when I measure the values the Rainfall value is still 0.2

I have the same problem when trying to zero the wind value.

What am I doing wrong here?

Thanks,

Paul.