Calling the UpdateSubscriptionItem API returns the error message: "Alignment CUS price total must be >= 0"

Hi,

one of our customers wants to downgrade the item quantity of their subscription from 10 to 4 by the next billing interval.

They are trying to call the UpdateSubscriptionItem API and execute the downgrade with the following:

curl --request POST
--url https://rest.cleverbridge.com/subscription/updatesubscriptionitem
--header 'accept: application/json'
--header 'content-type: application/json'
--header 'authorization: Basic xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
--data '{
"AlignmentSettings": {
"AlignToCurrentInterval": true,
"ExtendInterval": false,
"GetCustomerPricePreviewOnly": false
},
"GenerateMail": true,
"ProductId": 123456,
"Quantity": 4,
"RunningNumber": 1,
"SubscriptionId": "S12345678",
"TriggerImmediateRenewal": false,
"UpdateAction": "Downgrade"
}'

However, they keep getting the following error: "Alignment CUS price total must be >= 0"

How can they solve this?