diff --git a/pkg/store/bucket.go b/pkg/store/bucket.go
index 775210e3a1..6c38ee81a7 100644
--- a/pkg/store/bucket.go
+++ b/pkg/store/bucket.go
@@ -664,7 +664,7 @@ func debugFoundBlockSetOverview(logger log.Logger, mint, maxt int64, lset labels
 // TODO(bwplotka): It buffers all chunks in memory and only then streams to client.
 // 1. Either count chunk sizes and error out too big query.
 // 2. Stream posting -> series -> chunk all together.
-func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_SeriesServer) error {
+func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_SeriesServer) (err error) {
 	matchers, err := translateMatchers(req.Matchers)
 	if err != nil {
 		return status.Error(codes.InvalidArgument, err.Error())
@@ -731,6 +731,25 @@ func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_Serie
 
 	s.mtx.RUnlock()
 
+	defer func() {
+		s.metrics.seriesDataTouched.WithLabelValues("postings").Observe(float64(stats.postingsTouched))
+		s.metrics.seriesDataFetched.WithLabelValues("postings").Observe(float64(stats.postingsFetched))
+		s.metrics.seriesDataSizeTouched.WithLabelValues("postings").Observe(float64(stats.postingsTouchedSizeSum))
+		s.metrics.seriesDataSizeFetched.WithLabelValues("postings").Observe(float64(stats.postingsFetchedSizeSum))
+		s.metrics.seriesDataTouched.WithLabelValues("series").Observe(float64(stats.seriesTouched))
+		s.metrics.seriesDataFetched.WithLabelValues("series").Observe(float64(stats.seriesFetched))
+		s.metrics.seriesDataSizeTouched.WithLabelValues("series").Observe(float64(stats.seriesTouchedSizeSum))
+		s.metrics.seriesDataSizeFetched.WithLabelValues("series").Observe(float64(stats.seriesFetchedSizeSum))
+		s.metrics.seriesDataTouched.WithLabelValues("chunks").Observe(float64(stats.chunksTouched))
+		s.metrics.seriesDataFetched.WithLabelValues("chunks").Observe(float64(stats.chunksFetched))
+		s.metrics.seriesDataSizeTouched.WithLabelValues("chunks").Observe(float64(stats.chunksTouchedSizeSum))
+		s.metrics.seriesDataSizeFetched.WithLabelValues("chunks").Observe(float64(stats.chunksFetchedSizeSum))
+		s.metrics.resultSeriesCount.Observe(float64(stats.mergedSeriesCount))
+
+		level.Debug(s.logger).Log("msg", "stats query processed",
+			"stats", fmt.Sprintf("%+v", stats), "err", err)
+	}()
+
 	// Concurrently get data from all blocks.
 	{
 		span, _ := tracing.StartSpan(srv.Context(), "bucket_store_preload_all")
@@ -775,24 +794,6 @@ func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_Serie
 		stats.mergeDuration = time.Since(begin)
 		s.metrics.seriesMergeDuration.Observe(stats.mergeDuration.Seconds())
 	}
-
-	s.metrics.seriesDataTouched.WithLabelValues("postings").Observe(float64(stats.postingsTouched))
-	s.metrics.seriesDataFetched.WithLabelValues("postings").Observe(float64(stats.postingsFetched))
-	s.metrics.seriesDataSizeTouched.WithLabelValues("postings").Observe(float64(stats.postingsTouchedSizeSum))
-	s.metrics.seriesDataSizeFetched.WithLabelValues("postings").Observe(float64(stats.postingsFetchedSizeSum))
-	s.metrics.seriesDataTouched.WithLabelValues("series").Observe(float64(stats.seriesTouched))
-	s.metrics.seriesDataFetched.WithLabelValues("series").Observe(float64(stats.seriesFetched))
-	s.metrics.seriesDataSizeTouched.WithLabelValues("series").Observe(float64(stats.seriesTouchedSizeSum))
-	s.metrics.seriesDataSizeFetched.WithLabelValues("series").Observe(float64(stats.seriesFetchedSizeSum))
-	s.metrics.seriesDataTouched.WithLabelValues("chunks").Observe(float64(stats.chunksTouched))
-	s.metrics.seriesDataFetched.WithLabelValues("chunks").Observe(float64(stats.chunksFetched))
-	s.metrics.seriesDataSizeTouched.WithLabelValues("chunks").Observe(float64(stats.chunksTouchedSizeSum))
-	s.metrics.seriesDataSizeFetched.WithLabelValues("chunks").Observe(float64(stats.chunksFetchedSizeSum))
-	s.metrics.resultSeriesCount.Observe(float64(stats.mergedSeriesCount))
-
-	level.Debug(s.logger).Log("msg", "series query processed",
-		"stats", fmt.Sprintf("%+v", stats))
-
 	return nil
 }
 
@@ -1239,6 +1240,13 @@ func (p *postingGroup) Postings() index.Postings {
 		return index.EmptyPostings()
 	}
 
+	for i, posting := range p.postings {
+		if posting == nil {
+			// This should not happen. Debug for https://github.com/improbable-eng/thanos/issues/874.
+			return index.ErrPostings(errors.Errorf("at least one of %d postings is nil for %s. It was never fetched.", i, p.keys[i]))
+		}
+	}
+
 	return p.aggregate(p.postings)
 }
 
@@ -1327,6 +1335,7 @@ func (r *bucketIndexReader) fetchPostings(groups []*postingGroup) error {
 				continue
 			}
 
+			r.stats.postingsToFetch++
 			ptrs = append(ptrs, postingPtr{ptr: ptr, groupID: i, keyID: j})
 		}
 	}
@@ -1342,13 +1351,13 @@ func (r *bucketIndexReader) fetchPostings(groups []*postingGroup) error {
 	})
 
 	var g run.Group
-	for _, p := range parts {
+	for _, part := range parts {
 		ctx, cancel := context.WithCancel(r.ctx)
-		i, j := p.elemRng[0], p.elemRng[1]
+		i, j := part.elemRng[0], part.elemRng[1]
 
-		start := int64(p.start)
+		start := int64(part.start)
 		// We assume index does not have any ptrs that has 0 length.
-		length := int64(p.end) - start
+		length := int64(part.end) - start
 
 		// Fetch from object storage concurrently and update stats and posting list.
 		g.Add(func() error {
@@ -1697,6 +1706,7 @@ type queryStats struct {
 
 	postingsTouched          int
 	postingsTouchedSizeSum   int
+	postingsToFetch          int
 	postingsFetched          int
 	postingsFetchedSizeSum   int
 	postingsFetchCount       int