Incorrectly converts to a JSON string

I am writing an API that performs a query to the OSMR server and retrieves some data, tries to convert this data to JSON and return it in my API, everything works almost fine, but instead of numerical values ​​in the resulting string, it substitutes with the characters "{}". I have no idea what's going on, I've been working on this problem for an hour

package main
import (
"encoding/json"
"fmt"
"net/http"
"net/url"
"time"
"log"
"bytes"
"github.com/stretchr/objx"
"github.com/go-martini/martini"
)
func PrettyJson(data interface{}) (string) {
buffer := new(bytes.Buffer)
encoder := json.NewEncoder(buffer)
encoder.SetIndent("", "    ")
err := encoder.Encode(data)
if err != nil {
return ""
}
return buffer.String()
}
func main() {
app := martini.Classic()
app.Get("/", func() string {
return "/"
})
app.Get("/routes", func(req *http.Request, res http.ResponseWriter) string {
query, err := url.ParseQuery(req.URL.RawQuery)
if err != nil {
log.Println(err)
}
var client = &http.Client{Timeout: 100 * time.Second}
if v1, v2 := query["src"], query["dst"]; v1 != nil && v2 != nil {
var routes_array []map[string]interface{}
for _, destination := range query["dst"] {
request_url := fmt.Sprintf("http://router.project-osrm.org/route/v1/driving/%s;%s?overview=false", query["src"][0], destination)
response, err := client.Get(request_url)
if err != nil {
log.Fatal(err)
return "{\"message\": \"internal server error\"}"
}
buf := new(bytes.Buffer)
buf.ReadFrom(response.Body)
newStr := buf.String()
document, _ := objx.FromJSON(newStr)
route := map[string]interface{}{
"destination": destination,
"distance":    document.Get("routes[0].distance"),
"duration":    document.Get("routes[0].duration"),
}
routes_array = append(routes_array, route)
}
response := map[string]interface{} {
"source": query["src"],
"routes": routes_array,
}
fmt.Println("ROUTES_ARRAY")
fmt.Println(routes_array)
fmt.Println()
fmt.Println("RESPONSE")
fmt.Println(response)
fmt.Println()
return PrettyJson(response)
}
return "{\"message\": \"incorrect value of arguments\"}"
})
app.Run()
}

Output from my API

 {
"routes": [
{
"destination": "13.397634,52.529407",
"distance": {},
"duration": {}
},
{
"destination": "13.428555,52.523219",
"distance": {},
"duration": {}
}
],
"source": [
"13.388860,52.517037"
]
}

routes_array and response variable output

ROUTES_ARRAY
[map[destination:13.397634,52.529407 distance:1884.8 duration:251.4] 
map[destination:13.428555,52.523219 distance:3795.2 duration:384.5]]
RESPONSE
map[routes:[map[destination:13.397634,52.529407 distance:1884.8 duration:251.4] 
map[destination:13.428555,52.523219 distance:3795.2 duration:384.5]] source: 
[13.388860,52.517037]]

An objx.Value is marshaled to an empty object because the type does not have any exported fields.

Use the Data method to get the underlying value. Marshal the underlying value.

route := map[string]interface{}{
"destination": destination,
"distance":    document.Get("routes[0].distance").Data(),
"duration":    document.Get("routes[0].duration").Data(),
}