Error in family document: Modifying model outside of transaction

Revit
1

I am trying to turn on/off the Light Source parameter in families. But I keep getting this transaction error.

Warning: IronPythonEvaluator.EvaluateIronPythonScript operation failed. 
Traceback (most recent call last):
  File "<string>", line 28, in <module>
Exception: Attempt to modify the model outside of transaction.

import clr

clr.AddReference("RevitServices")
import RevitServices
from RevitServices.Persistence import DocumentManager 

clr.AddReference("RevitAPI")
import Autodesk 
from Autodesk.Revit.DB import *

currDoc = DocumentManager.Instance.CurrentDBDocument
uiapp = DocumentManager.Instance.CurrentUIApplication
app = uiapp.Application

# Current doc/app/ui
doc = DocumentManager.Instance.CurrentDBDocument

#Inputdoc
famDoc = IN[0]

trans = Transaction(doc, 'Light Source')
trans.Start()

OwnerFam = famDoc.OwnerFamily

lightSource = OwnerFam.get_Parameter(BuiltInParameter.FBX_ASSET_TYPE)
lightSource.Set(0)

trans.Commit()
TransactionManager.Instance.ForceCloseTransaction()

OUT = famDoc
3

Turns out I have to modify the line doc to famDoc

trans = Transaction(famDoc, 'Light Source')