Request can't be modified in closed status "The quantity cannot be changed"

Symptoms:
It's not possible to edit a closed request. Warning mentions a quantity field which is not available through Request interface.

Error/warning messages:
"The request is closed. The quantity cannot be changed."

Reason:
Quantity is linked to Work Request that has Billing type "Product and effort".

Request data have been uploaded in database and by default, Quantity field in Request table has been set to 0.

Resolution:
In order to avoid a check from Changepoint for quantity field, the field Quantity for Request table must be a NULL value by default, and not 0.