bleve highlight incorrect result

gistfile1.go

package main

import (
	"log"

	"github.com/blevesearch/bleve"
)

func main() {
	indexMapping := bleve.NewIndexMapping()

	index, err := bleve.NewUsing("boltdb", indexMapping, "boltdb", nil)
	if err == bleve.ErrorIndexPathExists {
		index, err = bleve.OpenUsing("boltdb", nil)
	}

	if err != nil {
		log.Fatalln(err)
	}

	docs := []struct {
		Title string
		Tags  []string
	}{
		{
			Title: "highlight",
			Tags:  []string{"abc", "123 45 678"},
		},
	}

	for _, doc := range docs {
		index.Index(doc.Title, doc)
	}

	keyword := "123"
	query := bleve.NewMatchQuery(keyword)
	search := bleve.NewSearchRequest(query)
	search.Highlight = bleve.NewHighlight()
	searchResults, err := index.Search(search)
	if err != nil {
		log.Fatalln(err)
		return
	}

	log.Printf("result: %s: %s \n", keyword, searchResults)
}