Applying shell operator to extruded dxf shape leads to 'BRep_API: command not done' error

While trying to create 3d models from dxf files where the top and bottom of the shapes are removed, I ran into a problem.
According to the documentation the shell operator does what I want, but when I apply it to an imported shape I get the following error:

```
~/miniconda3/lib/python3.8/site-packages/cadquery/cq.py in shell(self, thickness, kind)
1089 faces = [f for f in self.objects if isinstance(f, Face)]
1090
-> 1091 s = solidRef.shell(faces, thickness, kind=kind)
1092 return self.newObject([s])
1093

~/miniconda3/lib/python3.8/site-packages/cadquery/occ_impl/shapes.py in shell(self, faceList, thickness, tolerance, kind)
1860
1861 shell_builder.Build()
-> 1862 rv = shell_builder.Shape()
1863
1864 else: # if no faces provided a watertight solid will be constructed

StdFail_NotDone: BRep_API: command not done
```

This is the code I am using to generate the model:
```python
result = cq.importers.importDXF('test.dxf').wires().toPending().extrude(10).faces('|Z').shell(1)
```
And this is a zip file containing an example dxf file:
[test.zip](https://github.com/CadQuery/cadquery/files/5573038/test.zip)

I should mention that it works fine with very simple shapes like rectangles, but as soon as the shape is a little bit more complex I run into the error.

Is there something I am doing wrong or is this a bug?