Caption url returns empty string

I set caption:true when making generating video. In heygen GUI the captions are there. But the caption_url is empty string always.

result {
code: 100,
data: {
callback_id: null,
caption_url: '',
created_at: 1741894778,
duration: 9.788,
error: null,
gif_url: null,
id: '29b271bb66fd4462a8ae8aeaf482b11d',
status: 'completed',