Broken model with twistExtrude and fillet

Hey, I'm trying to make a simple pipe with twistExtrude, but the object I get is closed. Am I doing something wrong or is there a bug somewhere?

```
s1 = cq.Sketch().rect(10, 10).rect(9, 9, mode="s").vertices().fillet(1)
cq.Workplane("XY").placeSketch(s1).twistExtrude(60, 360)
```

Without `fillet` everything seems fine:

```
s1 = cq.Sketch().rect(10, 10).rect(9, 9, mode="s")
cq.Workplane("XY").placeSketch(s1).twistExtrude(60, 360)
```