diff --git a/pkg/store/hintspb/custom.go b/pkg/store/hintspb/custom.go
index 463418f6fe..40861143c0 100644
--- a/pkg/store/hintspb/custom.go
+++ b/pkg/store/hintspb/custom.go
@@ -37,7 +37,7 @@ func (m *QueryStats) Merge(other *QueryStats) {
 
 	m.SeriesFetched += other.SeriesFetched
 	m.SeriesFetchCount += other.SeriesFetchCount
-	m.SeriesFetchedSizeSum += m.SeriesFetchedSizeSum
+	m.SeriesFetchedSizeSum += other.SeriesFetchedSizeSum
 	m.SeriesTouched += other.SeriesTouched
 	m.SeriesTouchedSizeSum += other.SeriesTouchedSizeSum
 
diff --git a/pkg/store/hintspb/custom_test.go b/pkg/store/hintspb/custom_test.go
new file mode 100644
index 0000000000..3d9691f7c1
--- /dev/null
+++ b/pkg/store/hintspb/custom_test.go
@@ -0,0 +1,33 @@
+// Copyright (c) The Thanos Authors.
+// Licensed under the Apache License 2.0.
+
+package hintspb
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/efficientgo/core/testutil"
+)
+
+func TestQueryStatsMerge(t *testing.T) {
+	s := &QueryStats{}
+	ps := reflect.Indirect(reflect.ValueOf(s))
+	for i := 0; i < ps.NumField(); i++ {
+		ps.FieldByIndex([]int{i}).SetInt(int64(1))
+	}
+	o := &QueryStats{}
+	po := reflect.Indirect(reflect.ValueOf(o))
+	for i := 0; i < po.NumField(); i++ {
+		po.FieldByIndex([]int{i}).SetInt(int64(100))
+	}
+	s.Merge(o)
+
+	// Expected stats.
+	e := &QueryStats{}
+	pe := reflect.Indirect(reflect.ValueOf(e))
+	for i := 0; i < pe.NumField(); i++ {
+		pe.FieldByIndex([]int{i}).SetInt(int64(101))
+	}
+	testutil.Equals(t, e, s)
+}