Unset field with `import --set`

When using `modify` one can unset a field using `field!`. It is not possible to do so using the `set_fields` importer option nor the `--set` command line option.

```
error: supplied argument `subcollection!' is not of the form `key=value'
```

How can I unset a field during import using the the configuration or command line option? (I don't want to set it to the empty string).