Type error when loading custom grammar object

[TheOnlyTails]

I'm trying to load a custom language grammar into shiki like so:

import grammar from "../syntax/nymph.tmLanguage.json" with { type: "json" };

// ...
await shiki.loadLanguage(grammar);

but i'm getting an error, saying that the $self and $base properties are missing from IRawRepositoryMap.
However, looking at the official grammars package, I can't see those properties in any other grammar.