diff --git a/promql/engine_test.go b/promql/engine_test.go
index e6794f455b..ad3bb0513d 100644
--- a/promql/engine_test.go
+++ b/promql/engine_test.go
@@ -253,13 +253,23 @@ load 10s
 		{
 			Query: "metric",
 			Result: Matrix{Series{
-				Points: []Point{{V: 1, T: 0}, {V: 1, T: 1000}, {V: 2, T: 2000}},
+				Points: []Point{{V: 1, T: 0}, {V: 1, T: 1000}, {V: 1, T: 2000}},
 				Metric: labels.FromStrings("__name__", "metric")},
 			},
 			Start:    time.Unix(0, 0),
 			End:      time.Unix(2, 0),
 			Interval: time.Second,
 		},
+		{
+			Query: "metric",
+			Result: Matrix{Series{
+				Points: []Point{{V: 1, T: 0}, {V: 1, T: 5000}, {V: 2, T: 10000}},
+				Metric: labels.FromStrings("__name__", "metric")},
+			},
+			Start:    time.Unix(0, 0),
+			End:      time.Unix(10, 0),
+			Interval: 5 * time.Second,
+		},
 	}
 
 	for _, c := range cases {
diff --git a/storage/buffer.go b/storage/buffer.go
index d3687d94fd..0159cfa3fb 100644
--- a/storage/buffer.go
+++ b/storage/buffer.go
@@ -13,7 +13,9 @@
 
 package storage
 
-import "math"
+import (
+	"math"
+)
 
 // BufferedSeriesIterator wraps an iterator with a look-back buffer.
 type BufferedSeriesIterator struct {
@@ -21,6 +23,7 @@ type BufferedSeriesIterator struct {
 	buf *sampleRing
 
 	lastTime int64
+	ok       bool
 }
 
 // NewBuffer returns a new iterator that buffers the values within the time range
@@ -30,6 +33,7 @@ func NewBuffer(it SeriesIterator, delta int64) *BufferedSeriesIterator {
 		it:       it,
 		buf:      newSampleRing(delta, 16),
 		lastTime: math.MinInt64,
+		ok:       true,
 	}
 	it.Next()
 
@@ -56,8 +60,8 @@ func (b *BufferedSeriesIterator) Seek(t int64) bool {
 	if t0 > b.lastTime {
 		b.buf.reset()
 
-		ok := b.it.Seek(t0)
-		if !ok {
+		b.ok = b.it.Seek(t0)
+		if !b.ok {
 			return false
 		}
 		b.lastTime, _ = b.Values()
@@ -77,14 +81,19 @@ func (b *BufferedSeriesIterator) Seek(t int64) bool {
 
 // Next advances the iterator to the next element.
 func (b *BufferedSeriesIterator) Next() bool {
+	if !b.ok {
+		return false
+	}
+
 	// Add current element to buffer before advancing.
 	b.buf.add(b.it.At())
 
-	ok := b.it.Next()
-	if ok {
+	b.ok = b.it.Next()
+	if b.ok {
 		b.lastTime, _ = b.Values()
 	}
-	return ok
+
+	return b.ok
 }
 
 // Values returns the current element of the iterator.
diff --git a/storage/buffer_test.go b/storage/buffer_test.go
index 6855a1daea..a03081ab6a 100644
--- a/storage/buffer_test.go
+++ b/storage/buffer_test.go
@@ -137,6 +137,26 @@ func TestBufferedSeriesIterator(t *testing.T) {
 	require.False(t, it.Next(), "next succeeded unexpectedly")
 }
 
+// At() should not be called once Next() returns false.
+func TestBufferedSeriesIteratorNoBadAt(t *testing.T) {
+	done := false
+
+	m := &mockSeriesIterator{
+		seek: func(int64) bool { return false },
+		at: func() (int64, float64) {
+			require.False(t, done)
+			done = true
+			return 0, 0
+		},
+		next: func() bool { return !done },
+		err:  func() error { return nil },
+	}
+
+	it := NewBuffer(m, 60)
+	it.Next()
+	it.Next()
+}
+
 func BenchmarkBufferedSeriesIterator(b *testing.B) {
 	var (
 		samples []sample
@@ -165,16 +185,16 @@ func BenchmarkBufferedSeriesIterator(b *testing.B) {
 }
 
 type mockSeriesIterator struct {
-	seek   func(int64) bool
-	values func() (int64, float64)
-	next   func() bool
-	err    func() error
+	seek func(int64) bool
+	at   func() (int64, float64)
+	next func() bool
+	err  func() error
 }
 
-func (m *mockSeriesIterator) Seek(t int64) bool        { return m.seek(t) }
-func (m *mockSeriesIterator) Values() (int64, float64) { return m.values() }
-func (m *mockSeriesIterator) Next() bool               { return m.next() }
-func (m *mockSeriesIterator) Err() error               { return m.err() }
+func (m *mockSeriesIterator) Seek(t int64) bool    { return m.seek(t) }
+func (m *mockSeriesIterator) At() (int64, float64) { return m.at() }
+func (m *mockSeriesIterator) Next() bool           { return m.next() }
+func (m *mockSeriesIterator) Err() error           { return m.err() }
 
 type mockSeries struct {
 	labels   func() labels.Labels