When the field is NULL (which happens to be the defaultValue for MappedDateTime)

Message: java.lang.NullPointerException
net.liftweb.mapper.MappedDateTime.asJsonValue(MappedDateTime.scala:49)
com.foursquare.model.Venue$mayor_updated$.asJsonValue(Venue.scala:50001)
net.liftweb.mapper.BaseMappedField$class.asJsonField(MappedField.scala:92)
net.liftweb.mapper.MappedDateTime.asJsonField(MappedDateTime.scala:34)
com.foursquare.model.Venue$mayor_updated$.asJsonField(Venue.scala:50001)
net.liftweb.mapper.MetaMapper$$anonfun$encodeAsJSON_$bang$1$$anonfun$apply$14.apply(MetaMapper.scala:571)
net.liftweb.mapper.MetaMapper$$anonfun$encodeAsJSON_$bang$1$$anonfun$apply$14.apply(MetaMapper.scala:571)
scala.List.map(List.scala:812)
net.liftweb.mapper.MetaMapper$$anonfun$encodeAsJSON_$bang$1.apply(MetaMapper.scala:571)
net.liftweb.mapper.MetaMapper$$anonfun$encodeAsJSON_$bang$1.apply(MetaMapper.scala:568)