Nested map of struct doesn't ignore empty values

[co-in]

https://play.golang.org/p/1Wv3nOtX1CC

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"github.com/jinzhu/copier"
)

type mySubStruct struct {
	Val6 *string
	Val7 *string
}

type myStruct struct {
	Val1 *string
	Val2 *string
	Val3 *string
	Val4 map[string]mySubStruct
	Val5 map[string]*string
	Val8 *string
}

func main() {
	var err error
	m1 := new(myStruct)
	v1 := []byte(`{
		"val1":"####",
		"val2":null,
		"val3":"Tres",
		"val4":{
			"test":{
				"val6":"####",
				"val7":"Cinco"
			}
		},
		"val8":"Siete"
	}`)
	err = json.NewDecoder(bytes.NewReader(v1)).Decode(m1)
	if err != nil {
		panic(err)
	}

	m2 := new(myStruct)
	v2 := []byte(`{
		"val1":"Uno",
		"val2":"Dos",
		"val4":{
			"test":{
				"val6":"Cuatro"
			}
		},
		"val5":{
			"left":"Seis"
		},
		"val8":null
	}`)
	err = json.NewDecoder(bytes.NewReader(v2)).Decode(m2)
	if err != nil {
		panic(err)
	}

	err = copier.CopyWithOption(m1, m2, copier.Option{IgnoreEmpty: true, DeepCopy: true})

	b, _ := json.Marshal(m1)
	fmt.Println("Result\t", string(b))
	fmt.Println("Expected", `{"val1":"Uno","val2":"Dos","val3":"Tres","val4":{"test":{"val6":"Cuatro","val7":"Cinco"}},"val5":{"left":"Seis"}},"val8":"Siete"`)
}