**List of steps to reproduce** (step by step, including full links if applicable):
1. Make a page called `User:RheingoldRiver/Test` (or whatever sandbox page)
2. Give it contentmodel `text`.
Then this API query (I ran them in the API sandbox so it's definitely not a connection/library issue) gives an internal MediaWiki error (see below):
```
{
"action": "edit",
"format": "json",
"title": "User:RheingoldRiver/Test",
"appendtext": "asdf",
"contentmodel": "wikitext",
"token": (token here)
}
```
But this one (with `text`) will go through:
```
{
"action": "edit",
"format": "json",
"title": "User:RheingoldRiver/Test",
"text": "asdf",
"contentmodel": "text",
"token": (token here)
}
```
**What happens?**:
I get this error message:
```
{
"error": {
"code": "internal_api_error_MWException",
"info": "[436f0277b9c349e2] Caught exception of type MWException",
"errorclass": "MWException"
}
}
```
**What should have happened instead?**:
It should append the text & change the contentmodel to `wikitext`.
**Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc**:
* I discovered this is on a Fandom/Gamepedia wiki (https://lol.fandom.com) on MediaWiki 1.33.