Using 2 GIs in another GI

Hi @all,

I created 2 GIs:

1. List of budget per month (GLBudgetLineDetail) screenshot below
2. Sales per month (ARHistory) screenshot below

Now I created another GI and used my GIs above as my tables.

Then I Joined the two tabels, that’s ok.

But when I try to select the fields for my Join, I can’t choose the fields I need. screenshot below

In both GIs exist the field “FinPeriodID”. That should be my Join-Field, but it is not shown in Acumatica. I can’t choose it. 

Has anyone an idea, how this can be done?

 IMAGE 1 - List of budget per month

IMAGE 2 - Sales per month

IMAGE 3 - Can’t choose fields I need