JsonMappingException when grouping by host

[ibnesayeed]

I received the following exception when attempting to group crawl log by hosts:

$ java -jar target/*.jar -g host crawl.log > hosts.json
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?) (through reference chain: java.util.HashMap["null"])
	at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:285)
	at com.fasterxml.jackson.databind.SerializerProvider.mappingException(SerializerProvider.java:1251)
	at com.fasterxml.jackson.databind.SerializerProvider.reportMappingProblem(SerializerProvider.java:1145)
	at com.fasterxml.jackson.databind.ser.impl.FailingSerializer.serialize(FailingSerializer.java:35)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeOptionalFields(MapSerializer.java:781)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:639)
	at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:33)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
	at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3906)
	at com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:3177)
	at org.netpreserve.logtrix.CrawlSummary.main(CrawlSummary.java:211)