Can't create triggers with aggregated items

[eran]

When trying to create a trigger using an aggregated item the server gives an error "Expression ... does not match to [server:key.func(param)]"

For example:
Expression [{global:grpsum('Farm1','perf_counter[\Web Service(_Total)\Current Connections]','last','0').last(0)}] does not match to [server:key.func(param)]

If this is just a validation problem, how can I put it in the DB manually?

Thanks,
Eran