Error while adding new record without explicit key (using auto inc key instead)

Using the idb package I'm trying to add new records to my application DB. The object store is created like this:

const dbEditorStore = db.createObjectStore("myStore",
    { keyPath: "id", autoIncrement: true });
dbEditorStore.createIndex("resultIndex", "resultId", { unique: false });
    dbEditorStore.createIndex("tabIndex", "tabId", { unique: false });

When I now try to add new data, without explicitly giving a key I get the error

> ConstraintError: Key already exists in the object store.

Here's how I add the data (note the missing key parameter):

await db.add("myStore", data);

which confuses me as I expect that for an auto increment key I don't have to specify anything and IndexedDB would handle that automatically.

What is wrong here and what is the correct way to add data in this scenario?

Further info: when I specify the key name in the add call I get a different error:

> DataError: Failed to execute 'add' on 'IDBObjectStore': The object store uses in-line keys and the key parameter was provided.

An apparently unrelated answer about unmet conditions when inserting data gave me the ultimate idea what was wrong. When specifying the keyPath value no separate primary key is created, but the field given in the key path is used for the primary key (inline key). This aspect is badly described in the MDN documentation:

> key path
Defines where the browser should extract the key from in the object store or index. A valid key path can include one of the following: an empty string, a JavaScript identifier, or multiple JavaScript identifiers separated by periods or an array containing any of those. It cannot include spaces.

What is "the key" and how can a key be extracted from an object store?

Anyway, the solution here is to not specify a key at all:

const dbEditorStore = db.createObjectStore("myStore", { autoIncrement: true });

but only the key generator (autoIncrement). I assumed the value keyPath would be an automatic value (since it cannot be automatically changed, if it were referring to a field in the data).