Why do some functions need .bind() used with tryit() ?

[stefaanv]

I love tryit() dut don't understand why the functions that I pass sometimes need to be bound. For instance, the below code won't work without the .bind(em) part (findOne is from the mikro-orm package, this._em is an EntityManager object)

    const em = this._em.fork()
    const findOne = tryit(em.findOne.bind(em))
    const either = await findOne(IndexValue, { index: this._spotBelpex }, { orderBy: { startTime: 'DESC' } })

What's worse, adding the .bind(em) turns findOne from type

(entityName: EntityName<Entity>, ...) => Promise<...>

into type

(...args: any[]) => [Error, undefined] | [undefined, unknown]

which causes a warning on the await on the next line (the await not being necessary because the following function does not return a promise).
Strangely, the code with the .bind() and the warning works fine while the code without gives a runtime error.