diff --git a/cmd/tsdb/main.go b/cmd/tsdb/main.go
index fe84340c0..146aac08a 100644
--- a/cmd/tsdb/main.go
+++ b/cmd/tsdb/main.go
@@ -332,20 +332,22 @@ func exitWithError(err error) {
 	os.Exit(1)
 }
 
-func printBlocks(blocks []tsdb.DiskBlock) {
+func printBlocks(blocks []*tsdb.Block) {
 	tw := tsdb.GetNewTabWriter(os.Stdout)
 	defer tw.Flush()
 
 	fmt.Fprintln(tw, "BLOCK ULID\tMIN TIME\tMAX TIME\tNUM SAMPLES\tNUM CHUNKS\tNUM SERIES")
 	for _, b := range blocks {
+		meta := b.Meta()
+
 		fmt.Fprintf(tw,
 			"%v\t%v\t%v\t%v\t%v\t%v\n",
-			b.Meta().ULID,
-			b.Meta().MinTime,
-			b.Meta().MaxTime,
-			b.Meta().Stats.NumSamples,
-			b.Meta().Stats.NumChunks,
-			b.Meta().Stats.NumSeries,
+			meta.ULID,
+			meta.MinTime,
+			meta.MaxTime,
+			meta.Stats.NumSamples,
+			meta.Stats.NumChunks,
+			meta.Stats.NumSeries,
 		)
 	}
 }
diff --git a/head.go b/head.go
index 8f5beb4fb..e0fd3c076 100644
--- a/head.go
+++ b/head.go
@@ -74,6 +74,7 @@ type headMetrics struct {
 	series              prometheus.Gauge
 	seriesCreated       prometheus.Counter
 	seriesRemoved       prometheus.Counter
+	seriesNotFound      prometheus.Counter
 	chunks              prometheus.Gauge
 	chunksCreated       prometheus.Gauge
 	chunksRemoved       prometheus.Gauge
@@ -103,6 +104,10 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 		Name: "tsdb_head_series_removed_total",
 		Help: "Total number of series removed in the head",
 	})
+	m.seriesNotFound = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "tsdb_head_series_not_found",
+		Help: "Total number of requests for series that were not found.",
+	})
 	m.chunks = prometheus.NewGauge(prometheus.GaugeOpts{
 		Name: "tsdb_head_chunks",
 		Help: "Total number of chunks in the head block.",
@@ -149,6 +154,7 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 			m.series,
 			m.seriesCreated,
 			m.seriesRemoved,
+			m.seriesNotFound,
 			m.minTime,
 			m.maxTime,
 			m.gcDuration,
@@ -833,24 +839,17 @@ func (h *headIndexReader) SortedPostings(p Postings) Postings {
 	if err := p.Err(); err != nil {
 		return errPostings{err: errors.Wrap(err, "expand postings")}
 	}
-	var err error
 
 	sort.Slice(ep, func(i, j int) bool {
-		if err != nil {
-			return false
-		}
 		a := h.head.series.getByID(ep[i])
 		b := h.head.series.getByID(ep[j])
 
 		if a == nil || b == nil {
-			err = errors.Errorf("series not found")
+			level.Debug(h.head.logger).Log("msg", "looked up series not found")
 			return false
 		}
 		return labels.Compare(a.lset, b.lset) < 0
 	})
-	if err != nil {
-		return errPostings{err: err}
-	}
 	return newListPostings(ep)
 }
 
@@ -859,6 +858,7 @@ func (h *headIndexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkM
 	s := h.head.series.getByID(ref)
 
 	if s == nil {
+		h.head.metrics.seriesNotFound.Inc()
 		return ErrNotFound
 	}
 	*lbls = append((*lbls)[:0], s.lset...)
diff --git a/index.go b/index.go
index 89f776433..c0680aa33 100644
--- a/index.go
+++ b/index.go
@@ -525,6 +525,8 @@ type IndexReader interface {
 
 	// Postings returns the postings list iterator for the label pair.
 	// The Postings here contain the offsets to the series inside the index.
+	// Found IDs are not strictly required to point to a valid Series, e.g. during
+	// background garbage collections.
 	Postings(name, value string) (Postings, error)
 
 	// SortedPostings returns a postings list that is reordered to be sorted
@@ -533,6 +535,7 @@ type IndexReader interface {
 
 	// Series populates the given labels and chunk metas for the series identified
 	// by the reference.
+	// Returns ErrNotFound if the ref does not resolve to a known series.
 	Series(ref uint64, lset *labels.Labels, chks *[]ChunkMeta) error
 
 	// LabelIndices returns the label pairs for which indices exist.
diff --git a/querier.go b/querier.go
index 808a1057a..12e05e79d 100644
--- a/querier.go
+++ b/querier.go
@@ -450,6 +450,10 @@ Outer:
 	for s.p.Next() {
 		ref := s.p.At()
 		if err := s.index.Series(ref, &lset, &chunks); err != nil {
+			// Postings may be stale. Skip if no underlying series exists.
+			if errors.Cause(err) == ErrNotFound {
+				continue
+			}
 			s.err = err
 			return false
 		}
diff --git a/querier_test.go b/querier_test.go
index e46f3b947..b6f95bf7c 100644
--- a/querier_test.go
+++ b/querier_test.go
@@ -702,9 +702,8 @@ func TestBaseChunkSeries(t *testing.T) {
 					ref: 108,
 				},
 			},
-			postings: []uint64{12, 10, 108},
-
-			expIdxs: []int{0, 1, 3},
+			postings: []uint64{12, 13, 10, 108}, // 13 doesn't exist and should just be skipped over.
+			expIdxs:  []int{0, 1, 3},
 		},
 		{
 			series: []refdSeries{
@@ -718,12 +717,11 @@ func TestBaseChunkSeries(t *testing.T) {
 				{
 					lset:   labels.New([]labels.Label{{"b", "c"}}...),
 					chunks: []ChunkMeta{{Ref: 8282}},
-					ref:    1,
+					ref:    3,
 				},
 			},
 			postings: []uint64{},
-
-			expIdxs: []int{},
+			expIdxs:  []int{},
 		},
 	}