Cut hole in B-Spline-surface

Hello, I'm experimenting a bit with cutting a hole in a B-Spline surface. I'm facing the problem, that I don't get the surface with the hole. I use DRAW:

#load face s1 and cutter s2

restore s1.brep s1

restore s2.brep s2

#get wire for hole in parametric space of the surface

bop s1 s2

bopsection r

explode r E

wire w r_1 r_2 r_3 r_4

#get B-Spline-surface from s1

mksurface surf s1

#create trimmed face from original surface and wire

mkface s1trimmed surf w

#visualize result

vdisplay s1trimmed

Displaying the result only shows the wire. Why is that?