Почему не работает скрипт для модального окна?

<i class="fa fa-chevron-circle-left" aria-hidden="true"></i>
<i class="fa fa-chevron-circle-right" aria-hidden="true"></i>
<div id="myModal" class="modal">
       <span class="close">&times;</span>
       <img class="modal-content" id="img01"/>
</div>
<div class="gallery">
        <img class="img" src="images/interior_1.jpg" alt="#"/>
        <img class="img" src="images/interior_2.jpg" alt="#"/>
         <img class="img" src="images/interior_3.jpg" alt="#"/>
          <img class="img" src="images/interior_4.jpg" alt="#"/>
          <img class="img" src="images/interior_5.jpg" alt="#"/>
</div>

.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 5; /* Sit on top */
    padding-top: 100px; /* Location of the box */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    overflow: auto; /* Enable scroll if needed */
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.9); /* Black w/ opacity */
}

.modal-content {
    margin: auto;
    display: block;
    width: 80%;
    max-width: 700px;
    max-height: 500px;
}

.modal-content {    
    -webkit-animation-name: zoom;
    -webkit-animation-duration: 0.6s;
    animation-name: zoom;
    animation-duration: 0.6s;
}

@-webkit-keyframes zoom {
    from {-webkit-transform:scale(0)} 
    to {-webkit-transform:scale(1)}
}

@keyframes zoom {
    from {transform:scale(0)} 
    to {transform:scale(1)}
}

.close {
    position: absolute;
    top: 15px;
    right: 35px;
    color: #f1f1f1;
    font-size: 40px;
    font-weight: bold;
    transition: 0.3s;
}

.close:hover,
.close:focus {
    color: #bbb;
    text-decoration: none;
    cursor: pointer;
}

var modal = document.getElementById("myModal");

    // Get the image and insert it inside the modal
    var img = document.getElementsByClassName("img");
    var modalImg = document.getElementById("img01");
    img.onclick = function(){
        modal.style.display = "block";
        modalImg.src = this.src;
    };

    // Get the <span> element that closes the modal
    var span = document.getElementsByClassName("close")[0];

    // When the user clicks on <span> (x), close the modal
    span.onclick = function() { 
        modal.style.display = "none";
    };