diff --git a/pkg/storage/series.go b/pkg/storage/series.go
index 1626bade198..eb78128f31e 100644
--- a/pkg/storage/series.go
+++ b/pkg/storage/series.go
@@ -301,7 +301,7 @@ func (s *MemSeries) truncateChunksBefore(mint int64) (removed int) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
-	if s.timestamps[0].maxTime > mint {
+	if len(s.timestamps) == 0 || s.timestamps[0].maxTime > mint {
 		// We don't have anything to do and can exist early.
 		return 0
 	}
@@ -364,6 +364,13 @@ func (s *MemSeries) truncateChunksBefore(mint int64) (removed int) {
 	s.durations = s.durations[start:]
 	s.periods = s.periods[start:]
 
+	// Update the series' numSamples according to the number timestamps.
+	var numSamples uint16
+	for _, t := range s.timestamps {
+		numSamples += uint16(t.chunk.NumSamples())
+	}
+	s.numSamples = numSamples
+
 	for key, chunks := range s.cumulativeValues {
 		s.cumulativeValues[key] = chunks[start:]
 	}
diff --git a/pkg/storage/series_test.go b/pkg/storage/series_test.go
index df255e7e262..5debc9ea718 100644
--- a/pkg/storage/series_test.go
+++ b/pkg/storage/series_test.go
@@ -309,17 +309,18 @@ func TestMemSeries_truncateChunksBefore(t *testing.T) {
 	testcases := []struct {
 		before int64
 
-		truncated int
-		left      int
-		minTime   int64
-		maxTime   int64
+		truncated  int
+		left       int
+		minTime    int64
+		maxTime    int64
+		numSamples uint16
 	}{
-		{before: 10, truncated: 0, left: 5, minTime: 1, maxTime: 500},
-		{before: 50, truncated: 0, left: 5, minTime: 1, maxTime: 500},
-		{before: 123, truncated: 1, left: 4, minTime: 121, maxTime: 500},
-		{before: 256, truncated: 2, left: 3, minTime: 241, maxTime: 500},
-		{before: 490, truncated: 4, left: 1, minTime: 481, maxTime: 500},
-		{before: 1_000, truncated: 5, left: 0, minTime: math.MaxInt64, maxTime: math.MinInt64},
+		{before: 10, truncated: 0, left: 5, minTime: 1, maxTime: 500, numSamples: 500},
+		{before: 50, truncated: 0, left: 5, minTime: 1, maxTime: 500, numSamples: 500},
+		{before: 123, truncated: 1, left: 4, minTime: 121, maxTime: 500, numSamples: 380},
+		{before: 256, truncated: 2, left: 3, minTime: 241, maxTime: 500, numSamples: 260},
+		{before: 490, truncated: 4, left: 1, minTime: 481, maxTime: 500, numSamples: 20},
+		{before: 1_000, truncated: 5, left: 0, minTime: math.MaxInt64, maxTime: math.MinInt64, numSamples: 500},
 	}
 
 	chunkPool := newHeadChunkPool()
@@ -346,6 +347,7 @@ func TestMemSeries_truncateChunksBefore(t *testing.T) {
 
 			require.Equal(t, tc.minTime, s.minTime)
 			require.Equal(t, tc.maxTime, s.maxTime)
+			require.Equal(t, tc.numSamples, s.numSamples)
 
 			require.Equal(t, tc.left, len(s.timestamps))
 			require.Equal(t, tc.left, len(s.durations))