diff --git a/tsdb/querier.go b/tsdb/querier.go
index b4513218e..74c73f460 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -958,6 +958,7 @@ type mergedStringIter struct {
 	b        index.StringIter
 	aok, bok bool
 	cur      string
+	err      error
 }
 
 func (m *mergedStringIter) Next() bool {
@@ -968,29 +969,34 @@ func (m *mergedStringIter) Next() bool {
 	if !m.aok {
 		m.cur = m.b.At()
 		m.bok = m.b.Next()
+		m.err = m.b.Err()
 	} else if !m.bok {
 		m.cur = m.a.At()
 		m.aok = m.a.Next()
+		m.err = m.a.Err()
 	} else if m.b.At() > m.a.At() {
 		m.cur = m.a.At()
 		m.aok = m.a.Next()
+		m.err = m.a.Err()
 	} else if m.a.At() > m.b.At() {
 		m.cur = m.b.At()
 		m.bok = m.b.Next()
+		m.err = m.b.Err()
 	} else { // Equal.
 		m.cur = m.b.At()
 		m.aok = m.a.Next()
+		m.err = m.a.Err()
 		m.bok = m.b.Next()
+		if m.err == nil {
+			m.err = m.b.Err()
+		}
 	}
 
 	return true
 }
 func (m mergedStringIter) At() string { return m.cur }
 func (m mergedStringIter) Err() error {
-	if m.a.Err() != nil {
-		return m.a.Err()
-	}
-	return m.b.Err()
+	return m.err
 }
 
 // DeletedIterator wraps chunk Iterator and makes sure any deleted metrics are not returned.
diff --git a/tsdb/querier_bench_test.go b/tsdb/querier_bench_test.go
index 1feb48f6f..78993d105 100644
--- a/tsdb/querier_bench_test.go
+++ b/tsdb/querier_bench_test.go
@@ -19,9 +19,10 @@ import (
 	"strconv"
 	"testing"
 
-	"github.com/stretchr/testify/require"
-
 	"github.com/prometheus/prometheus/model/labels"
+	"github.com/prometheus/prometheus/tsdb/index"
+
+	"github.com/stretchr/testify/require"
 )
 
 // Make entries ~50B in size, to emulate real-world high cardinality.
@@ -202,6 +203,28 @@ func benchmarkLabelValuesWithMatchers(b *testing.B, ir IndexReader) {
 	}
 }
 
+func BenchmarkMergedStringIter(b *testing.B) {
+	numSymbols := 100000
+	s := make([]string, numSymbols)
+	for i := 0; i < numSymbols; i++ {
+		s[i] = fmt.Sprintf("symbol%v", i)
+	}
+
+	for i := 0; i < b.N; i++ {
+		it := NewMergedStringIter(index.NewStringListIter(s), index.NewStringListIter(s))
+		for j := 0; j < 100; j++ {
+			it = NewMergedStringIter(it, index.NewStringListIter(s))
+		}
+
+		for it.Next() {
+			require.NotNil(b, it.At())
+			require.NoError(b, it.Err())
+		}
+	}
+
+	b.ReportAllocs()
+}
+
 func BenchmarkQuerierSelect(b *testing.B) {
 	chunkDir := b.TempDir()
 	opts := DefaultHeadOptions()