When agent down,How to stop an item generate data that type is Calculated

[hjfeng1988]

I have an item and it's type is Calculated
Item name:Used memory in %
Item formula:100-last("vm.memory.size[pavailable]")
When my agent down,It's still generating data，I guess that just because last("vm.memory.size[pavailable]") get the last data before agent down.How can i stop item generate new data.