Aff `get_coefficients_by_name` intended behavior?

```python
import islpy
aff = islpy.Aff("[N] -> { [x] -> [x+1] } ")
aff.get_coefficients_by_name(dimtype=None)
```
gives
> isl_aff.c:763: output/set dimension does not have a coefficient

According the doc, setting `dimtype` to None should produce the coefficients for all dimensions?
I digged a little, in the generated _isl.py,
```
_CHECK_DIM_TYPES = [
dim_type.in_, dim_type.param, dim_type.set]
```
which is used by `obj_get_coefficients_by_name` to retrieve the coefficients. When does Aff has a set dimension? Is this a bug or intended behavior?