Formula syntax - last divided by average

[DembeanCadillac]

I'm trying to create an item that divides the last asterisk.pjsip.available by its 12-hour average, however, I seem to not understand the syntax well.

last(//asterisk.pjsip.available)/avg(//asterisk.pjsip.available,12h)

The above formula allows me to save the Item, but then I receive an error "Cannot evaluate function "last()": item "ast://asterisk.pjsip.available" does not exist."

I checked the item "Asterisk: PJSIP available endpoints" and the key listed there is asterisk.pjsip.available. What did I do wrong please? Could this be because asterisk.pjsip.available is a dependent item?

[cyber]

Shouldn't there be hostname in empty //-s... otherwise it should work without issues...
func(/host/item,<params>)