Tenant property won't accept a variable

I’ve put my AWS account ID into a seperate yaml file, excluded it from my Git repo and referenced it as so:
custom: accountID: ${file(./account.yml):accountID}
I don’t seem able to do this for my tenant property value:
tenant: ${file(./account.yml):tenant}
equally if I try to do this indirectly as below it still isn’t recognised
tenant: ${self:custom.tenant}
Any thoughts