why I can't pass the param AnimationEvent.objectReferenceParameter,it's always null in even

code is simple just like this:

var evt = new AnimationEvent(); evt.time = eventTime; evt.functionName = eventName; evt.objectReferenceParameter = data; clip.AddEvent(evt);

data is base on UnityEngine.Object.I log objectReferenceParameter in event,it`s aways null.any one know the reason?