diff --git a/storage/metric/tiered/memory.go b/storage/metric/tiered/memory.go
index 76b2ff1ada..2015a35147 100644
--- a/storage/metric/tiered/memory.go
+++ b/storage/metric/tiered/memory.go
@@ -17,6 +17,8 @@ import (
 	"sort"
 	"sync"
 
+	"github.com/golang/glog"
+
 	clientmodel "github.com/prometheus/client_golang/model"
 
 	"github.com/prometheus/prometheus/storage/metric"
@@ -54,10 +56,26 @@ func (s *arrayStream) metric() clientmodel.Metric {
 	return s.m
 }
 
+// add implements the stream interface. This implementation requires both
+// s.values and the passed in v to be sorted already. Values in v that have a
+// timestamp older than the most recent value in s.values are skipped.
 func (s *arrayStream) add(v metric.Values) {
 	s.Lock()
 	defer s.Unlock()
-
+	// Skip over values that are older than the most recent value in s.
+	if len(s.values) > 0 {
+		i := 0
+		mostRecentTimestamp := s.values[len(s.values)-1].Timestamp
+		for ; i < len(v) && mostRecentTimestamp > v[i].Timestamp; i++ {
+		}
+		if i > 0 {
+			glog.Warningf(
+				"Skipped out-of-order values while adding to %#v: %#v",
+				s.m, v[:i],
+			)
+			v = v[i:]
+		}
+	}
 	s.values = append(s.values, v...)
 }
 
diff --git a/storage/metric/tiered/memory_test.go b/storage/metric/tiered/memory_test.go
index 19a157880e..1d45cbca43 100644
--- a/storage/metric/tiered/memory_test.go
+++ b/storage/metric/tiered/memory_test.go
@@ -15,6 +15,7 @@ package tiered
 
 import (
 	"fmt"
+	"reflect"
 	"runtime"
 	"sync"
 	"testing"
@@ -49,6 +50,55 @@ func BenchmarkStreamAdd(b *testing.B) {
 	b.Logf("%d cycles with %f bytes per cycle, totalling %d", b.N, float32(post.TotalAlloc-pre.TotalAlloc)/float32(b.N), post.TotalAlloc-pre.TotalAlloc)
 }
 
+func TestStreamAdd(t *testing.T) {
+	s := newArrayStream(clientmodel.Metric{})
+	// Add empty to empty.
+	v := metric.Values{}
+	expected := metric.Values{}
+	s.add(v)
+	if got := s.values; !reflect.DeepEqual(expected, got) {
+		t.Fatalf("Expected values %#v in stream, got %#v.", expected, got)
+	}
+	// Add something to empty.
+	v = metric.Values{
+		metric.SamplePair{Timestamp: 1, Value: -1},
+	}
+	expected = append(expected, v...)
+	s.add(v)
+	if got := s.values; !reflect.DeepEqual(expected, got) {
+		t.Fatalf("Expected values %#v in stream, got %#v.", expected, got)
+	}
+	// Add something to something.
+	v = metric.Values{
+		metric.SamplePair{Timestamp: 2, Value: -2},
+		metric.SamplePair{Timestamp: 5, Value: -5},
+	}
+	expected = append(expected, v...)
+	s.add(v)
+	if got := s.values; !reflect.DeepEqual(expected, got) {
+		t.Fatalf("Expected values %#v in stream, got %#v.", expected, got)
+	}
+	// Add something outdated to something.
+	v = metric.Values{
+		metric.SamplePair{Timestamp: 3, Value: -3},
+		metric.SamplePair{Timestamp: 4, Value: -4},
+	}
+	s.add(v)
+	if got := s.values; !reflect.DeepEqual(expected, got) {
+		t.Fatalf("Expected values %#v in stream, got %#v.", expected, got)
+	}
+	// Add something partially outdated to something.
+	v = metric.Values{
+		metric.SamplePair{Timestamp: 3, Value: -3},
+		metric.SamplePair{Timestamp: 6, Value: -6},
+	}
+	expected = append(expected, metric.SamplePair{Timestamp: 6, Value: -6})
+	s.add(v)
+	if got := s.values; !reflect.DeepEqual(expected, got) {
+		t.Fatalf("Expected values %#v in stream, got %#v.", expected, got)
+	}
+}
+
 func benchmarkAppendSamples(b *testing.B, labels int) {
 	b.StopTimer()
 	s := NewMemorySeriesStorage(MemorySeriesOptions{})