Selected faces must be co-planar

Hi,
I have the following code snippet:
```
def perFace(face: cq.Workplane):
return (
face.box(2, 2, 2)
)
part = (
cq.Workplane("XY")
.polygon(6, (radius + wall + shift)*2, circumscribed=True)
.extrude(-partHeight)
)
part = (
part.faces("#Z")
.workplane()
.each(perFace)
)
```
I am trying to apply the box to each side face of the polygon, but I get the error `ValueError: Selected faces must be co-planar`
How can I repeat the operation in the function `perFace` for each selected face? `faces("#Z").workplane().each(perFace)` seems not to work 🤔