Fillet fails when using #Z selector but *not* when all edges selected

I'm trying to get a better understanding of selectors so I wrote a test of various selectors:

```
import cadquery as cq
from ocp_vscode import *

def make_shape():
return (
cq.Workplane("XY")
.box(50, 10, 10)
.box(40, 20, 20)
.box(30, 30, 30)
.box(20, 20, 40)
.box(10, 10, 50)
.box(10, 50, 10)
.box(20, 40, 20)
)

for selector in [">Z", " {len(list(example))} edges")
try:
example = example.fillet(1)
except:
print(f"{selector} failed")
show(example)
cq.exporters.export(example, f"Tests/edge_test/selector_{selector}.stl")
```

The behavior is puzzling. For the "#Z" perpendicular selector only, I get this on the fillet command:

`OCP.OCP.StdFail.StdFail_NotDone: BRep_API: command not done`

What puzzles me is that if I use "" as a selector (selecting all edges) then cadquery perfectly happily fillets every single edge.

Shouldn't it then be possible to fillet all of the edges "#Z" selects, which should be a subset?