Pokemon past abilities not returning correct data

When getting the details of a pokemon (pokeapi.co/api/v2/pokemon/{id}) the past_abilities are wrong and the response is always:

"past_abilities": [
  {
    "abilities": [
      {
        "ability": null,
        "is_hidden": true,
        "slot": 3
      }
    ],
    "generation": {
      "name": "generation-iv",
      "url": "https://pokeapi.co/api/v2/generation/4/"
    }
  }
]

The null ability breaks the deserialization contract because a null ability should not exist.
I've been able to reproduce the issue with the first 10 pokemon (id 1 to 10).

Steps to Reproduce:

1. call https://pokeapi.co/api/v2/pokemon/1
2. scroll to the past_abilities data

Start by calling the documented Pokémon endpoint for IDs 1 through 10 and inspect how past_abilities is assembled in the API. Trace the source of the null ability entries, then verify that these responses no longer include null abilities and retain the correct past ability data.