WebRTCStats - GoogRTT and PacketsLost is null sometimes

Question

Just wondering why this might be the case.

Looking over an entire media session between two WebRTC peers, it seems that some reports from the 'send' audio channel have null values for googRtt and packetsLost. Is there a reason for this?

Example:

"sendAudioChannel": {
            "googCodecName": "opus",
            "googRtt": null,
            "bytesSent": 22636,
            "packetsSent": 190,
            "packetsLost": null,
            "audioInputLevel": 0,
            "googJitterReceived": null,
            "googResidualEchoLikelihood": 0,
            "googEchoCancellationReturnLoss": -100,
            "googVoiceCaptureToPacketDelayMs": 0,
            "googEchoCancellationReturnLossEnhancement": -100
          }

This event is from half way through the call.