Losing units when summing a Waveform

I multiplied two Waveform objects: one containing voltages and the other containing currents. Then I multiplied it by a PeriodValue. As expected, the resulting Wavform object contains numbers with the units of Joules. But when I applied the sum() method to this Waveform, I got a scalar value with no units.

Is there any way to do this without losing the units for the calculations that follow?