Kapacitor Join using 'on' Property and Single Dimension

Question

I'm joining streams in Kapacitor using the on property. The join only seems to work if one of the streams has multiple groupBy dimensions, even if only one dimension is actually needed. Why is that?

For example, in the code below, the join won't return anything if floor was removed from .groupBy('building', 'floor'). Why doesn't the join work with building alone?

var building = stream
    |from()
        .measurement('building_power')
        .groupBy('building')
var floor = stream
    |from()
        .measurement('floor_power')
        .groupBy('building', 'floor')
building
    |join(floor)
        .as('building', 'floor')
        .on('building')