Why do firebase collections seem to begin with a null row?

I've created a simple firebase data set to test some REST calls with (see the image below). I'm wondering why, when I query the collections in the database, firebase always return a null row first and then the actual rows. Here is what I get when I export the data. Notice the null rows under systems and system_types:

{
  "systems" : [ null, {
    "system_type_id" : 2,
    "name" : "Commodore 128",
    "id" : 1
  }, {
    "system_type_id" : 1,
    "name" : "Difference Engine",
    "id" : 2
  }, {
    "system_type_id" : 2,
    "name" : "Osborne",
    "id" : 3
  } ],
  "system_types" : [ null, {
    "name" : "Babbage",
    "id" : 1
  }, {
    "name" : "Von Neumann",
    "id" : 2
  } ]
}