diff --git a/grpc/federation/cel/url.go b/grpc/federation/cel/url.go
index a1c0495e..05c39cf1 100644
--- a/grpc/federation/cel/url.go
+++ b/grpc/federation/cel/url.go
@@ -2,11 +2,12 @@ package cel
 
 import (
 	"context"
+	"net/url"
+
 	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
 	"github.com/google/cel-go/common/types/traits"
-	"net/url"
 )
 
 const URLPackageName = "url"
@@ -381,6 +382,9 @@ func (lib *URLLibrary) CompileOptions() []cel.EnvOption {
 					}
 
 					v.Path, err = url.JoinPath(v.Path, paths...)
+					if err != nil {
+						return types.NewErr(err.Error())
+					}
 
 					return lib.toURLValue(v)
 				},
@@ -448,10 +452,6 @@ func (lib *URLLibrary) CompileOptions() []cel.EnvOption {
 					queryParams := v.Query()
 					queryMap := map[ref.Val]ref.Val{}
 					for key, values := range queryParams {
-						var valueList []ref.Val
-						for _, val := range values {
-							valueList = append(valueList, types.String(val))
-						}
 						queryMap[types.String(key)] = types.NewStringList(adapter, values)
 					}
 
diff --git a/grpc/federation/cel/url_test.go b/grpc/federation/cel/url_test.go
index dc68a38f..809c2ba9 100644
--- a/grpc/federation/cel/url_test.go
+++ b/grpc/federation/cel/url_test.go
@@ -3,8 +3,6 @@ package cel_test
 import (
 	"context"
 	"fmt"
-	"github.com/google/cel-go/common/types/traits"
-	"github.com/google/go-cmp/cmp/cmpopts"
 	"net/url"
 	"strings"
 	"testing"
@@ -12,7 +10,10 @@ import (
 	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+
 	cellib "github.com/mercari/grpc-federation/grpc/federation/cel"
 )
 
@@ -425,7 +426,7 @@ func TestURLFunctions(t *testing.T) {
 			},
 		},
 		{
-			name: "join path",
+			name: "url join path",
 			expr: `grpc.federation.url.parse('https://example.com/path?query=1#fragment').joinPath(['/new'])`,
 			cmp: func(got ref.Val) error {
 				gotV, ok := got.Value().(*cellib.URL)