Lookup in average summary field

I have a grid application with a group by with some summary totals of type average.
The original field is of type integer and the summary field is of type integer.

I have a lookup on the original field and that works ok.
I have set up to do lookup in both grid and summary in the setting for the field in the grid section, but I don’t get a lookup on the average in the summary.

It is not possible to enter a lookup for the summary fields in the group by section or the summary section.

Do you have any ideas for solving this?