I try to use the FETCH API to handover Data to a php Programm. But in the JS console i keep getting this error.

SyntaxError: Unexpected token '<', "

<br />
<b>"... is not valid JSON

const category = document.getElementById('genreSelect').value;
const js1 = '{"genre": "' + category + '"}';
const js2 = JSON.stringify(js1); 

fetch("button.php", {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json'
                },
                body: { category: js2 }
            })
            .then(response => response.json())
            .then(data => {

               

                
                

            })
            .catch(error => {
                console.error('Error:', error);
            });

<?php

    header('Content-Type: application/json');

   
    

    if(isset($_POST)){
        $data = file_get_contents('php://input');

        $kate = json_decode($data, true);
        $kat = $kate[0];


        $every = array();

echo json_encode($every);

I validated the JSON, but it was fine, so I ran out of possibilities left to do.