【JavaScript】スクロール位置を取得してクラスを付与したい(classList)

Uncaught TypeError: Cannot read properties of null (reading 'classList')

<header id="header">
        <div class="header_in">
            <h1>
                <a href="#"><img src="images/top-header-logo.png" alt="ロゴ画像"></a>
            </h1>
            <nav>
                <ul>
                    <li>お部屋</li>
                    <li>お料理</li>
                    <li>温泉</li>
                </ul>
            </nav>
            <div class="header_btn">
                <button type="button">宿泊予約</button>
            </div>
        </div>
    </header>

header{
    width: 100%;
    position: fixed;
    z-index: 100;
    top: 0;
}
header.scroll_{
    background-color: #fff;
    height: 80px;
}

window.onscroll = scroll;

function scroll(){
    scroll_position = window.pageYOffset;
    console.log( scroll_position );
    if(scroll_position >= 150){
        document.getElementById('header').classList.add('scroll_');
    }else{
        document.getElementById('header').classList.remove('scroll_');
    };
}