diff --git a/pkg/oidc/types.go b/pkg/oidc/types.go
index 8423cff..86e5d06 100644
--- a/pkg/oidc/types.go
+++ b/pkg/oidc/types.go
@@ -60,8 +60,8 @@ type ResponseType string
 
 type Scopes []string
 
-func (s *Scopes) Encode() string {
-	return strings.Join(*s, " ")
+func (s Scopes) Encode() string {
+	return strings.Join(s, " ")
 }
 
 func (s *Scopes) UnmarshalText(text []byte) error {
diff --git a/pkg/rp/relaying_party.go b/pkg/rp/relaying_party.go
index a8bb9bb..6807221 100644
--- a/pkg/rp/relaying_party.go
+++ b/pkg/rp/relaying_party.go
@@ -5,10 +5,12 @@ import (
 	"errors"
 	"net/http"
 	"net/url"
+	"reflect"
 	"strings"
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/gorilla/schema"
 
 	"github.com/caos/oidc/pkg/oidc"
 	"github.com/caos/oidc/pkg/oidc/grants"
@@ -24,6 +26,16 @@ const (
 	jwtProfileKey = "urn:ietf:params:oauth:grant-type:jwt-bearer"
 )
 
+var (
+	encoder = func() utils.Encoder {
+		e := schema.NewEncoder()
+		e.RegisterEncoder(oidc.Scopes{}, func(value reflect.Value) string {
+			return value.Interface().(oidc.Scopes).Encode()
+		})
+		return e
+	}()
+)
+
 //RelayingParty declares the minimal interface for oidc clients
 type RelayingParty interface {
 	//OAuthConfig returns the oauth2 Config
@@ -334,7 +346,7 @@ func CallTokenEndpoint(request interface{}, rp RelayingParty) (newToken *oauth2.
 }
 
 func callTokenEndpoint(request interface{}, authFn interface{}, rp RelayingParty) (newToken *oauth2.Token, err error) {
-	req, err := utils.FormRequest(rp.OAuthConfig().Endpoint.TokenURL, request, authFn)
+	req, err := utils.FormRequest(rp.OAuthConfig().Endpoint.TokenURL, request, encoder, authFn)
 	if err != nil {
 		return nil, err
 	}
diff --git a/pkg/utils/http.go b/pkg/utils/http.go
index e785472..fa51815 100644
--- a/pkg/utils/http.go
+++ b/pkg/utils/http.go
@@ -10,8 +10,6 @@ import (
 	"net/url"
 	"strings"
 	"time"
-
-	"github.com/gorilla/schema"
 )
 
 var (
@@ -36,9 +34,8 @@ func AuthorizeBasic(user, password string) RequestAuthorization {
 	}
 }
 
-func FormRequest(endpoint string, request interface{}, authFn interface{}) (*http.Request, error) {
+func FormRequest(endpoint string, request interface{}, encoder Encoder, authFn interface{}) (*http.Request, error) {
 	form := url.Values{}
-	encoder := schema.NewEncoder()
 	if err := encoder.Encode(request, form); err != nil {
 		return nil, err
 	}