Align-items: почему не работает?

<div class="register-container-flex">
        <div class="register-container">
        </div>
    </div>

.register-container-flex{
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
}

.register-container{
    width: 100%;
    max-width: 500px;
    height: 600px;
    background-color: #0000F0;
}