"Iteration over a unidirectional join is not supported" ??

With the following I get the error "Iteration over a unidirectional join is not supported".

      select
        avg(w.value)
      from
        Wattage as w unidirectional,
        BState.std:lastevent() as b
      where
        b = true
      output
        snapshot every 1 min

What I'm trying to do is to create an average over an interval but only accept Wattage events when a certain (other) state is true.