Uniqueid: <null> Questions

I have some questions regarding OriginateResponse, I have some case that if I have a Channel is busy. I got the following OriginateResponse

Event: Hangup
Privilege: call,all
Channel: SIP/2000-000000bf
Uniqueid: 1307971909.209
CallerIDNum: 123456
CallerIDName:
Cause: 17
Cause-txt: User busy

Event: OriginateResponse
Privilege: call,all
ActionID: 896607825_2#
Response: Failure
Channel: sip/2000
Context: dtmfcmd_01
Exten: 12345
Reason: 5
Uniqueid: // my question is why in this case the Uniqueid is
CallerIDNum: 123456
CallerIDName:

Thanks