How to use slices of custom type in sessions?

session_example.go

package main

import (
	"encoding/gob"
	"fmt"
	"github.com/gorilla/mux"
	"github.com/gorilla/sessions"
	"net/http"
)

type Person struct {
	Name string
	Age  string
}

var store = sessions.NewCookieStore([]byte("foobar"))
var persons []Person

func main() {
	r := mux.NewRouter()
	sub := r.Methods("GET", "POST").Subrouter()
	sub.HandleFunc("/person/{name}/{age:[0-9]+}", PostHandler)
	sub.HandleFunc("/person/", GetHandler)
	srv := &http.Server{
		Handler: r,
		Addr:    "127.0.0.1:8080",
	}
	srv.ListenAndServe()
}

func init() {
	gob.Register(&Person{})
}

func PostHandler(w http.ResponseWriter, r *http.Request) {
	session, err := store.Get(r, "spamspam")
	if err != nil {
		fmt.Fprintf(w, "invalid session: %+v\n", err)
	}
	vars := mux.Vars(r)
	persons = append(persons, Person{Name: vars["name"], Age: vars["age"]})
	fmt.Fprintf(w, "persons: %+v\n", persons)
	session.Values["persons"] = persons
	session.Save(r, w)
}

func GetHandler(w http.ResponseWriter, r *http.Request) {
	session, err := store.Get(r, "spamspam")
	if err != nil {
		fmt.Fprintf(w, "invalid session: %+v\n", err)
	}

	val := session.Values["persons"]
	// What type is the session value `val`?
	if persons, ok := val.([]Person); ok {
		for _, person := range persons {
			fmt.Fprintln(w, person)
		}
	} else {
		fmt.Fprintf(w, "invalid type detected: %+v\n", ok)
	}
}