diff --git a/go.mod b/go.mod
index 7a4629b..c933ff3 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module github.com/VictoriaMetrics/metrics
 
 require github.com/valyala/histogram v1.0.1
+
+go 1.13
diff --git a/set.go b/set.go
index 293a46b..33a39a0 100644
--- a/set.go
+++ b/set.go
@@ -14,9 +14,10 @@ import (
 //
 // Set.WritePrometheus must be called for exporting metrics from the set.
 type Set struct {
-	mu sync.Mutex
-	a  []*namedMetric
-	m  map[string]*namedMetric
+	mu        sync.Mutex
+	a         []*namedMetric
+	m         map[string]*namedMetric
+	summaries []*Summary
 }
 
 // NewSet creates new set of metrics.
@@ -32,6 +33,9 @@ func (s *Set) WritePrometheus(w io.Writer) {
 		return s.a[i].name < s.a[j].name
 	}
 	s.mu.Lock()
+	for _, sm := range s.summaries {
+		sm.updateQuantiles()
+	}
 	if !sort.SliceIsSorted(s.a, lessFunc) {
 		sort.Slice(s.a, lessFunc)
 	}
@@ -200,6 +204,9 @@ func (s *Set) NewSummaryExt(name string, window time.Duration, quantiles []float
 	s.registerMetric(name, sm)
 	registerSummary(sm)
 	s.registerSummaryQuantiles(name, sm)
+	s.mu.Lock()
+	s.summaries = append(s.summaries, sm)
+	s.mu.Unlock()
 	return sm
 }
 
@@ -258,6 +265,7 @@ func (s *Set) GetOrCreateSummaryExt(name string, window time.Duration, quantiles
 			registerSummary(sm)
 			mustRegisterQuantiles = true
 		}
+		s.summaries = append(s.summaries, sm)
 		s.mu.Unlock()
 		if mustRegisterQuantiles {
 			s.registerSummaryQuantiles(name, sm)
diff --git a/summary.go b/summary.go
index 7075aab..41454e4 100644
--- a/summary.go
+++ b/summary.go
@@ -93,9 +93,9 @@ func (sm *Summary) UpdateDuration(startTime time.Time) {
 }
 
 func (sm *Summary) marshalTo(prefix string, w io.Writer) {
-	// Just update sm.quantileValues and don't write anything to w.
+	// Do nothing. Quantile values should be already updated by the caller
+	// via sm.updateQuantiles() call.
 	// sm.quantileValues will be marshaled later via quantileValue.marshalTo.
-	sm.updateQuantiles()
 }
 
 func (sm *Summary) updateQuantiles() {
diff --git a/summary_test.go b/summary_test.go
index ceaffa7..007a417 100644
--- a/summary_test.go
+++ b/summary_test.go
@@ -29,10 +29,10 @@ func TestSummarySerial(t *testing.T) {
 	}
 
 	// Make sure the summary doesn't print anything on marshalTo call
-	// and updates s.quantileValues.
 	testMarshalTo(t, s, "prefix", "")
 
 	// Verify s.quantileValues
+	s.updateQuantiles()
 	if s.quantileValues[len(s.quantileValues)-1] != 1999 {
 		t.Fatalf("unexpected quantileValues[last]; got %v; want %v", s.quantileValues[len(s.quantileValues)-1], 1999)
 	}