diff --git a/head.go b/head.go
index 8f5beb4fb6..1d0f688395 100644
--- a/head.go
+++ b/head.go
@@ -833,24 +833,17 @@ 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)
 }
 
diff --git a/index.go b/index.go
index 89f7764331..c0680aa338 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 808a1057aa..12e05e79df 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 e46f3b9475..b6f95bf7ce 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{},
 		},
 	}