Stone Portal from Ashlands update has a null "prefabName":

[b-morgan]

According to the Wiki (which may not be the most reliable source) it should be "portal_stone". I have a work around (see #99 for Python code) if this isn't something you can (should?) fix.

The zdoList item for a Stone Portal is below for reference.

{
    "persistent" : true,
    "type" : 2,
    "distant" : false,
    "prefab" : 1854482458,
    "prefabName" : null,
    "sector" : {
      "x" : 0,
      "y" : -2
    },
    "position" : {
      "x" : -10.132895,
      "y" : 31.679173,
      "z" : -111.687416
    },
    "rotation" : {
      "x" : -0.0,
      "y" : 292.5,
      "z" : 0.0
    },
    "floats" : { },
    "floatsByName" : {
      "health" : 413.77856,
      "support" : 1000.0
    },
    "vector3s" : null,
    "vector3sByName" : null,
    "quats" : null,
    "quatsByName" : null,
    "ints" : null,
    "intsByName" : null,
    "longs" : { },
    "longsByName" : {
      "creator" : 1615010464
    },
    "strings" : {
      "-1565613777" : "Steam_76561197985429755"
    },
    "stringsByName" : {
      "tag" : "Stone"
    },
    "byteArrays" : null,
    "byteArraysByName" : null,
    "saveConnections" : true,
    "withRotation" : true,
    "connectionType" : 17,
    "connectionHash" : 1911213956
  },