Group aggregation: "Exception while formatting value [...]"

Hey folks,

I am trying to setup a group for all of my temperature and humidity sensors and would like to derive a group state from the member items:

Group:Number:AVG        humidity        <humidity>
Group:Number:AVG        temperature     <temperature>

The items are defined as follows:

Number:Temperature    sensor_temperature_1  "Temperature [%.2f %unit%]" [...]
Number:Temperature    sensor_temperature_2  "Temperature [%.2f %unit%]" [...]
Number:Temperature    sensor_temperature_3  "Temperature [%.2f %unit%]" [...]

Number:Dimensionless  sensor_humidity_1     "Humidity [%.2f %unit%]"    [...]
Number:Dimensionless  sensor_humidity_2     "Humidity [%.2f %unit%]"    [...]
Number:Dimensionless  sensor_humidity_3     "Humidity [%.2f %unit%]"    [...]

The sensor I am using is the Ruuvi Tag, which I have added as a thing through the current binding. The values for both temperature and humidity are displayed fine in my sitemap. I can convert them as needed too.

Unfortunately, deriving the group state does not seem to work properly, as I am getting an error message:

2020-09-28 20:20:08.954 [WARN ] [ui.internal.items.ItemUIRegistryImpl] - Exception while formatting value ‘48.6250’ of item humidity with format ‘%.1f %unit%’: Conversion = ‘u’
2020-09-28 20:20:08.971 [WARN ] [ui.internal.items.ItemUIRegistryImpl] - Exception while formatting value ‘19.89’ of item temperature with format ‘%.1f %unit%’: Conversion = ‘u’
2020-09-28 20:20:09.502 [WARN ] [ui.internal.items.ItemUIRegistryImpl] - Exception while formatting value ‘48.6275’ of item humidity with format ‘%.1f %unit%’: Conversion = ‘u’
2020-09-28 20:20:09.513 [WARN ] [ui.internal.items.ItemUIRegistryImpl] - Exception while formatting value ‘19.89’ of item temperature with format ‘%.1f %unit%’: Conversion = ‘u’

This thread is dicussing the same problem, but with a different binding and its almost 5 years old:
https://community.openhab.org/t/items-in-sitemap-generate-log-message-exception-while-formatting-value-of-item/5401

The Group is of a different type to its members.
Try -
Group:Number:Dimensionless:AVG humidity ...

That did indeed do the trick. Thank you!

I have double checked the documentation, and unfortunately, I cannot find a reference to the solution you have proposed. Maybe it should be added to the documentation?