diff --git a/backend/union/upstream/upstream.go b/backend/union/upstream/upstream.go
index 93b8ae521..86575c512 100644
--- a/backend/union/upstream/upstream.go
+++ b/backend/union/upstream/upstream.go
@@ -6,8 +6,6 @@ import (
 	"fmt"
 	"io"
 	"math"
-	"path"
-	"path/filepath"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -91,7 +89,7 @@ func New(ctx context.Context, remote, root string, cacheTime time.Duration) (*Fs
 		return nil, err
 	}
 	f.RootFs = rFs
-	rootString := path.Join(remote, filepath.ToSlash(root))
+	rootString := fspath.JoinRootPath(remote, root)
 	myFs, err := cache.Get(ctx, rootString)
 	if err != nil && err != fs.ErrorIsFile {
 		return nil, err