diff --git a/internal/archiver/tree_test.go b/internal/archiver/tree_test.go
index 5d460bb37..00d0b6b82 100644
--- a/internal/archiver/tree_test.go
+++ b/internal/archiver/tree_test.go
@@ -1,6 +1,7 @@
 package archiver
 
 import (
+	"fmt"
 	"path/filepath"
 	"runtime"
 	"testing"
@@ -10,6 +11,9 @@ import (
 	restictest "github.com/restic/restic/internal/test"
 )
 
+// debug.Log requires Tree.String.
+var _ fmt.Stringer = Tree{}
+
 func TestPathComponents(t *testing.T) {
 	var tests = []struct {
 		p       string