Shell won't work if all but one faces are selected.

Shell would build for 2 faces of extruded rect but wont build if leave only one face.

Its not much like a big practical problem, because I can draw thin rect instead of shell in this case, but I think it could work for consistency.

~~~python
import cadquery as cq

res = (
cq.Workplane("XY")
.rect(4, 9)
.extrude(3)
#.faces(">Y or X or Y or X or Z") # fail
.faces(">Y or Z") # ok
.shell(-0.5)
)

show_object(res)
~~~