That's also heppened if you try to init new Swiper without slides or swiper-wrapper. I mean

<div class="swiper">
    <div class="swiper-wrapper"></div>
</div> 
<script>
    new Swiper('.swiper', {...})
</script>

OR

<div class="swiper">

</div>    
<script>
    new Swiper('.swiper', {...})
</script>