Inner shell is built for extruded polyline solid but outer shell is not built for same solid

I have solid extruded from polyline. If I make a shell for it with inner thickness (-0.5), it is built ok. If try to build outer shell (0.5 thickness), I get "Brep_API: command not done".

Maybe there is a reason for it, but looks like a bug or missing feature to me.

~~~python
import cadquery as cq

_plate = (
cq.Workplane("XY")
.lineTo(3, 0)
.lineTo(5, 1)
.lineTo(6, 1)
.lineTo(6, 4)
.lineTo(0, 4)
.mirrorY()
.extrude(3)
.faces("Z")
.shell(-0.5) # ok
#.shell(0.5) # fail
)

show_object(_plate)
~~~