POST in Reactjs with Object included, how to do?

Question

I need to register for course instructors, in which each instructor will have a course. How do I?

I’m doing it this way and it’s not going:

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

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 registerInstrutor, it gives me the error:

Typeerror: Cannot read Property '1' of Undefined

specifically when trying to save the variable stroke