Make sure you have the correct call. Don't put the &JSON in quotes after the ai_dump parameter.
{ "ai_dump":&JSON, "Image":"&ALERT_JPEG", "camera":"&CAM", "ALERT_PATH": "&ALERT_PATH", "ALERT_CLIP": "&ALERT_CLIP", "timestamp":"&ALERT_TIME" }
It looks like your &JSON is empty.
If still having...