Type conversion doesn't handle explicit nulls

[ianp]

If I have a field which may be null then it blows up on extraction:

val json = """{ "url": null }""".parseJson
json.extract[String]("url".?)

This causes a runtime exception with root cause:

spray.json.DeserializationException: Expected String as JsString, but got null

Is this an issue with son-lenses, or am I doing something wrong (a strong possibility)?