Specifying Persistence for {item}.previousState

I was having some problems from .perviousState using rrd4j so I tried moving to db4o, but it does not look like {item}.previousState is using the persistence type.

rule "Propane Used"
Item TankUtility_Tank changed
val double current_meter = TankUtility_Tank.state
val double previous_meter = TankUtility_Tank.previousState(true "db4o").state
val double today_propane_used = if(Today_Propane_Used.state == Uninitialized) 0 else Today_Propane_Used.state
postUpdate(Today_Propane_Used, today_propane_used + ((previous_meter - current_meter)*10))

So I think I am telling it to use db4o, but I still see the same error I saw with rrd4j:

2015-12-17 14:54:00.430 [ERROR] [o.o.c.s.ScriptExecutionThread ] - Error during the execution of rule 'Propane Used': rrd4j does not allow querys without a begin date, unless order is descending and a single value is requested

Probably not the cause, but you should have a command between true and "db4o" in your call to previousState.