Holoviews widget disappear in parameterized class

Panel
1 
import param
import panel as pn
import holoviews as hv
hv.extension('bokeh')

pn.Pane(hv.HoloMap({'a': hv.Points((0, 1)), 'b': hv.Points([])}))

displays just fine with the HoloMap widget, whereas with the code below, the widget is gone. Can anybody tell me why?

class Test(param.Parameterized):
    def plot(self):
        return pn.Pane(hv.HoloMap({'a': hv.Points((0, 1)), 'b': hv.Points([])}))
t = Test()
pn.Row(t.plot)