Can't edit PhotonVoiceRecorder.Transmit value

Im trying to implement a push-to-talk system to the Photon Voice Demo but it doesn't work.
Using this line : "GetComponent ().Transmit = true;" in Start() or even OnJoinedVoiceRoom() give me this error :
NullReferenceException: Object reference not set to an instance of an object
PhotonVoiceRecorder.set_Transmit (Boolean value) (at Assets/PUNVoice/Scripts/PhotonVoiceRecorder.cs:273)

It seems that it works when I create a room, but i always get this error if I join a room.

If someone could help me that would be great