--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -2357,28 +2357,18 @@ TrackBuffersManager::SkipToNextRandomAcc
   uint32_t parsed = 0;
   auto& trackData = GetTracksData(aTrack);
   const TrackBuffer& track = GetTrackBuffer(aTrack);
   aFound = false;
 
   // SkipToNextRandomAccessPoint can only be called if aTimeThreadshold is known
   // to be buffered.
 
-  // So first determine the current position in the track buffer if necessary.
-  if (trackData.mNextGetSampleIndex.isNothing()) {
-    if (trackData.mNextSampleTimecode == TimeUnit()) {
-      // First demux, get first sample.
-      trackData.mNextGetSampleIndex = Some(0u);
-    } else {
-      int32_t pos = FindCurrentPosition(aTrack, aFuzz);
-      if (pos < 0) {
-        return 0;
-      }
-      trackData.mNextGetSampleIndex = Some(uint32_t(pos));
-    }
+  if (NS_FAILED(SetNextGetSampleIndexIfNeeded(aTrack, aFuzz))) {
+    return 0;
   }
 
   TimeUnit nextSampleTimecode = trackData.mNextSampleTimecode;
   TimeUnit nextSampleTime = trackData.mNextSampleTime;
   uint32_t i = trackData.mNextGetSampleIndex.ref();
   int32_t originalPos = i;
 
   for (; i < track.Length(); i++) {
@@ -2634,18 +2624,23 @@ TrackBuffersManager::Evictable(TrackInfo
   return GetTracksData(aTrack).mEvictionIndex.mEvictable;
 }
 
 TimeUnit
 TrackBuffersManager::GetNextRandomAccessPoint(TrackInfo::TrackType aTrack,
                                               const TimeUnit& aFuzz)
 {
   MOZ_ASSERT(OnTaskQueue());
+
+  // So first determine the current position in the track buffer if necessary.
+  if (NS_FAILED(SetNextGetSampleIndexIfNeeded(aTrack, aFuzz))) {
+    return TimeUnit::FromInfinity();
+  }
+
   auto& trackData = GetTracksData(aTrack);
-  MOZ_ASSERT(trackData.mNextGetSampleIndex.isSome());
   const TrackBuffersManager::TrackBuffer& track = GetTrackBuffer(aTrack);
 
   uint32_t i = trackData.mNextGetSampleIndex.ref();
   TimeUnit nextSampleTimecode = trackData.mNextSampleTimecode;
   TimeUnit nextSampleTime = trackData.mNextSampleTime;
 
   for (; i < track.Length(); i++) {
     const MediaRawData* sample =
@@ -2657,16 +2652,55 @@ TrackBuffersManager::GetNextRandomAccess
       return sample->mTime;
     }
     nextSampleTimecode = sample->mTimecode + sample->mDuration;
     nextSampleTime = sample->GetEndTime();
   }
   return TimeUnit::FromInfinity();
 }
 
+nsresult
+TrackBuffersManager::SetNextGetSampleIndexIfNeeded(TrackInfo::TrackType aTrack,
+                                                   const TimeUnit& aFuzz)
+{
+  auto& trackData = GetTracksData(aTrack);
+  const TrackBuffer& track = GetTrackBuffer(aTrack);
+
+  if (trackData.mNextGetSampleIndex.isSome()) {
+    // We already know the next GetSample index.
+    return NS_OK;
+  }
+
+  if (!track.Length()) {
+    // There's nothing to find yet.
+    return NS_ERROR_DOM_MEDIA_END_OF_STREAM;
+  }
+
+  if (trackData.mNextSampleTimecode == TimeUnit()) {
+    // First demux, get first sample.
+    trackData.mNextGetSampleIndex = Some(0u);
+    return NS_OK;
+  }
+
+  if (trackData.mNextSampleTimecode >
+      track.LastElement()->mTimecode + track.LastElement()->mDuration) {
+    // The next element is past our last sample. We're done.
+    trackData.mNextGetSampleIndex = Some(uint32_t(track.Length()));
+    return NS_ERROR_DOM_MEDIA_END_OF_STREAM;
+  }
+
+  int32_t pos = FindCurrentPosition(aTrack, aFuzz);
+  if (pos < 0) {
+    // Not found, must wait for more data.
+    return NS_ERROR_DOM_MEDIA_WAITING_FOR_DATA;
+  }
+  trackData.mNextGetSampleIndex = Some(uint32_t(pos));
+  return NS_OK;
+}
+
 void
 TrackBuffersManager::TrackData::AddSizeOfResources(MediaSourceDecoder::ResourceSizes* aSizes) const
 {
   for (const TrackBuffer& buffer : mBuffers) {
     for (const MediaRawData* data : buffer) {
       aSizes->mByteSize += data->SizeOfIncludingThis(aSizes->mMallocSizeOf);
     }
   }

--- a/dom/media/mediasource/TrackBuffersManager.h
+++ b/dom/media/mediasource/TrackBuffersManager.h
@@ -155,16 +155,25 @@ public:
                                        const media::TimeUnit& aFuzz,
                                        bool& aFound);
 
   already_AddRefed<MediaRawData> GetSample(TrackInfo::TrackType aTrack,
                                            const media::TimeUnit& aFuzz,
                                            MediaResult& aResult);
   int32_t FindCurrentPosition(TrackInfo::TrackType aTrack,
                               const media::TimeUnit& aFuzz) const;
+
+  // Will set the next GetSample index if needed. This information is determined
+  // through the value of mNextSampleTimecode. Return false if the index
+  // couldn't be determined or if there's nothing more that could be demuxed.
+  // This occurs if either the track buffer doesn't contain the required
+  // timecode or is empty.
+  nsresult SetNextGetSampleIndexIfNeeded(TrackInfo::TrackType aTrack,
+                                         const media::TimeUnit& aFuzz);
+
   media::TimeUnit GetNextRandomAccessPoint(TrackInfo::TrackType aTrack,
                                            const media::TimeUnit& aFuzz);
 
   void AddSizeOfResources(MediaSourceDecoder::ResourceSizes* aSizes) const;
 
 private:
   typedef MozPromise<bool, MediaResult, /* IsExclusive = */ true> CodedFrameProcessingPromise;