Round plot with 'npstere'-projection

Dear all,

using a polarstereograhic projection with Basemap (

m = Basemap(projection = 'npstere', resolution = 'c', lat_0 = 90., lon_0 = 0.,
area_thresh = 1000., boundinglat = 40.

)

one get a square map. Is there any way to make the map round, not square?

Many thanks,