The valid values for next_episode_to_air and last_episode_to_air are either an object (an episode object) or null if there is no value.

Request

curl -v "http://api.themoviedb.org/3/tv/94997?api_key=###"

* Host api.themoviedb.org:80 was resolved.
* IPv6: (none)
* IPv4: 52.85.151.48, 52.85.151.24, 52.85.151.18, 52.85.151.28
*   Trying 52.85.151.48:80...
* Connected to api.themoviedb.org (52.85.151.48) port 80
> GET /3/tv/94997?api_key=### HTTP/1.1
> Host: api.themoviedb.org
> User-Agent: curl/8.6.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json;charset=utf-8
< Content-Length: 3503
< Connection: keep-alive
< Date: Fri, 19 Jul 2024 15:42:09 GMT
< Server: openresty
< Cache-Control: public, max-age=9426
< x-memc: HIT
< x-memc-key: c7ca821d28a8ce837bcb75dfaf156f97
< x-memc-age: 17164
< x-memc-expires: 9426
< ETag: W/"59380c9fc93fb145697457bc4d5a795f"
< Vary: Accept-Encoding,accept-encoding
< X-Cache: Miss from cloudfront
< Via: 1.1 08f45c153a856ff7955174d0e6f60744.cloudfront.net (CloudFront)
< X-Amz-Cf-Pop: IAD89-C3
< Alt-Svc: h3=":443"; ma=86400
< X-Amz-Cf-Id: 3xSafXW1pYCO4OIqXVdvIaDCpwBv7CK1cTInbkRJ4c569KmjVwSqiQ==
< Vary: Origin
<
* Connection #0 to host api.themoviedb.org left intact

Response (truncated)

{
    ...
    "id": 94997,
    "last_episode_to_air": {
        "id": 5234726,
        "name": "Regent",
        "overview": "Amid whispers of bad omens, the Greens consider how to fill a void on Aegon's Council, Jacaerys sets out on a rogue mission to strike a deal, and Daemon enlists Lord Willem Blackwood to help persuade the Brackens to bend the knee.",
        "vote_average": 7.4,
        "vote_count": 17,
        "air_date": "2024-07-14",
        "episode_number": 5,
        "episode_type": "standard",
        "production_code": "",
        "runtime": 63,
        "season_number": 2,
        "show_id": 94997,
        "still_path": "/b2ynXBWVy0dSL47lUDbQeiPW5SH.jpg"
    },
    "name": "House of the Dragon",
    "next_episode_to_air": {
        "id": 5234727,
        "name": "Episode 6",
        "overview": "",
        "vote_average": 0,
        "vote_count": 0,
        "air_date": "2024-07-21",
        "episode_number": 6,
        "episode_type": "standard",
        "production_code": "",
        "runtime": null,
        "season_number": 2,
        "show_id": 94997,
        "still_path": "/cCUY9j9Yv8q0T9Pw6oAzoMcbP1M.jpg"
    },
    ...
}

Where are you seeing it be anything else?

https://developer.themoviedb.org/reference/tv-series-details - response 200 - description of fields