Using JAS 0.16.13 on MC 1.7.10.
Adding a PostSpawn tag in EntityHandler configurations results in a crash.

Example that causes the crash:

"MoCreatures.Bird": {
  "Type-Enabled": "OPENSKY-true",
  "Spawn Operand": "OR",
  "Spawn Tag": "!sky()",
  "Despawn Tags": "isTamed()||block({'iron_bars'},{6,6,6},{0,0,0})",
  "PostSpawn Tags": "writenbt('TypeInt/3/4')",
  "Contents": [
    "MoCreatures.Bird"
  ]
},

Unlike issue 108, adding "PostSpawn Tags" = "true" does not cause the crash.
The above without the PostSpawn tags spawns the birds normally.

Crash: http://pastebin.com/EVtcC1ZV

Unfortunately now (with 0.16.14) I'm getting the exact same crash as above, except it also happens when setting PostSpawn tags in the SpawnListEntries file as well.

Setting "PostSpawn Tags": "true" in EntityHandler still works.

You're calling writenbt(String) which doesn't exist.

The only acceptable argument is with a String array: writeNBT(String[])

Arrays are marked with {} in MVEL. [] denotes a list which should also work: writenbt('TypeInt/3/4') needs to be writenbt({'TypeInt/3/4'})