Beast mode adding two columns with conditions

Hello Dojo,

I'm having trouble adding two columns with in a conditional case. The following works fine:

SUM(`paid_total`) + SUM(`booked_total`)

But then when I try to add any sort of condition, it fails: "An issue has occurred during processing. We are unable to complete the request at this time."

case
when `payment_number` >= 1
then SUM(`paid_total`) + SUM(`booked_total`)
else 0
end

I've tried wrapping the whole statement in a SUM as it suggests to do in the docs, but that also doesn't work.

SUM(
case
when `payment_number` >= 1
then SUM(`paid_total`) + SUM(`booked_total`)
else 0
end
)

Any thoughts here? I've exhausted every other help page on this topic I could find in the Dojo, to no avail.