POST in ReactJS with object included, how to do?

I need to make an enrollment for Course Instructors, in which each instructor will have a course. How do I?

I'm doing it that way and it's not going:

PS.: variable name and email receive a variable in String , already the course variable receives the course object.

Code:

cadastraInstrutor(name, email, curso) {
    return fetch('http://localhost:3001/instrutores/',{
        method: "POST",
        body: {name: name, email: email, curso: curso},
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        }
    })
}

After calling the function registerInstructor, it gives me the error:

  

TypeError: Can not read property '1' of undefined

Specifically when attempting to save the course variable