The key “_username” must be a string, “NULL” given

I’m receiving ‘The key "_username" must be a string, "NULL" given’ after submitting login form

this is my jsx:

fetch('/signin', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
    },
    body: JSON.stringify({
        _username: "test@example.com", // Test email
        _password: "your_password" // Test password
    })
})
.then((res) => res.json())

and my controller:

#[Route('/signin', name: 'app_login')]
public function login(AuthenticationUtils $authenticationUtils): Response
{
    if ($this->getUser()) {
        return $this->redirectToRoute('app_dashboard');
    }


    $error = $authenticationUtils->getLastAuthenticationError();
    $lastUsername = $authenticationUtils->getLastUsername();

    return $this->json([
        'last_username' => $lastUsername,
        'error' => $error ? $error->getMessage() : null,
    ]

and my security.yaml

        form_login:
            login_path: /signin
            check_path: /signin
            enable_csrf: true
            secure: auto

Can you please help me with that

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