Result of a combination sensor as area temperature

I have several KNX temperature sensors in one room. I configured a combination sensor to get the mean value of them as the room temperature. The combination sensor shows the correct value, but when I go to the area configuration to choose the sensor for the area temperature, I cannot choose the combination sensor just created.

What am I doing wrong?

The combo sensor might not be a number or it may need a unit of measure or something like that.
Are you getting an error message?