--- a/dom/media/encoder/TrackEncoder.cpp
+++ b/dom/media/encoder/TrackEncoder.cpp
@@ -296,16 +296,17 @@ VideoTrackEncoder::AppendVideoSegment(co
         continue;
       }
 
       // This is the first real chunk in the track. Use its timestamp as the
       // starting point for this track.
       MOZ_ASSERT(!chunk.mTimeStamp.IsNull());
       const StreamTime nullDuration = mLastChunk.mDuration;
       mLastChunk = chunk;
+      chunk.mDuration = 0;
 
       TRACK_LOG(LogLevel::Verbose,
                 ("[VideoTrackEncoder]: Got first video chunk after %lld ticks.",
                  nullDuration));
       // Adapt to the time before the first frame. This extends the first frame
       // from [start, end] to [0, end], but it'll do for now.
       CheckedInt64 diff = FramesToUsecs(nullDuration, mTrackRate);
       MOZ_ASSERT(diff.isValid());