Zabbix marks calculated item as "Not supported" if expression contains item that currently has no value. It would be nice to change that behavior to something more usable.
Example:
Current values for items:
item[/var] = 10
item[/home] = 20
item[/tmp] = 30
item[/usr] = UNDEFINED/NO LAST VALUE

Calculated item:
last("item[/var]")+last("item[/home]")+last("item[/tmp]")+last("item[/usr]") currently will returns UNDEFINED, while it makes sense to return 60

In other words UNDEFINED value for item should be considered as "0" in addition and subtraction operations and as "1" in multiply and division operations