correct declaration for Buttons

response.go

@@ -508,7 +508,7 @@ type StructuredMessageElement struct {
 	ItemURL       string                     `json:"item_url,omitempty"`
 	Subtitle      string                     `json:"subtitle"`
 	DefaultAction *DefaultAction             `json:"default_action,omitempty"`
-	Buttons       *[]StructuredMessageButton `json:"buttons"`
+	Buttons       *[]StructuredMessageButton `json:"buttons,omitempty"`
 	ReceiptMessageElement
 }
 

response_test.go

@@ -0,0 +1,17 @@
+package messenger
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_MarshalStructuredMessageElement(t *testing.T) {
+	data, err := json.Marshal(StructuredMessageElement{
+		Title: "Title",
+	})
+	require.NoError(t, err)
+	assert.JSONEq(t, string(data), `{"image_url":"", "subtitle":"", "title": "Title"}`)
+}