compability with json api

Hi,

I install plugin Json API https://www.ads-software.com/plugins/json-api/, but it detect thumbnail_image is null.
How to resolve this?

Respon example: https://localhost/?json=get_post&id=1924
:
thumbnail_image is null.
comment_status: “open”,
thumbnail: null,
custom_fields: {
tj_video_embed: [
“<iframe src="https://player.vimeo.com/video/48523509&quot; width="500" height="375" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> <p>PERMATA DI TENGAH DANAU | TRAILER from midfilms on Vimeo.</p>”
],
tweet_trim_url_2: [
“https://localhost/?p=1924&#8221;
],
tj_video_img_url: [
“https://localhost/wp-content/themes/videoplus/functions/thumb.php?src=http%3A%2F%2Fi.vimeocdn.com%2Fvideo%2F334963382_640.jpg&w=216&h=120&zc=1&#8221;
]
},
thumbnail_size: “thumbnail”,
thumbnail_images: [ ]

Thanks.