diff --git a/src/log/slog/level_test.go b/src/log/slog/level_test.go
index 0b28e71e4c..575df099da 100644
--- a/src/log/slog/level_test.go
+++ b/src/log/slog/level_test.go
@@ -5,6 +5,7 @@
 package slog
 
 import (
+	"bytes"
 	"flag"
 	"strings"
 	"testing"
@@ -52,10 +53,14 @@ func TestLevelVar(t *testing.T) {
 
 func TestMarshalJSON(t *testing.T) {
 	want := LevelWarn - 3
+	want_data := []byte{'"', 'I', 'N', 'F', 'O', '+', '1', '"'}
 	data, err := want.MarshalJSON()
 	if err != nil {
 		t.Fatal(err)
 	}
+	if !bytes.Equal(data, want_data) {
+                t.Errorf("got %s, want %s", string(data), string(want_data))
+        }
 	var got Level
 	if err := got.UnmarshalJSON(data); err != nil {
 		t.Fatal(err)
@@ -67,10 +72,14 @@ func TestMarshalJSON(t *testing.T) {
 
 func TestLevelMarshalText(t *testing.T) {
 	want := LevelWarn - 3
+	want_data := []byte{'I', 'N', 'F', 'O', '+', '1'}
 	data, err := want.MarshalText()
 	if err != nil {
 		t.Fatal(err)
 	}
+	if !bytes.Equal(data, want_data) {
+                t.Errorf("got %s, want %s", string(data), string(want_data))
+        }
 	var got Level
 	if err := got.UnmarshalText(data); err != nil {
 		t.Fatal(err)