heading_level is null under table of contents

I am using the api from datalab.to to transfer the pdf to json, by using 2 pdf book but both got the heading_level null

` {
        "title": "Experience Matters",
        "heading_level": null,
        "page_id": 12,
        "polygon": [
          [
`

`
{
        "title": "Ticket Server",
        "heading_level": null,
        "page_id": 147,
        "polygon": [
          [
`