Captions returning 'undefined' until first slide change

[anta199]

I'm trying to show captions for each image in a slider inside a modal.
It all works good but the caption returns 'undefined' until I slide for the first time – after that it works fine.

I assume it can be related with the initialisation of the slider and the modal?
Thanks

Here's my code so far:

    var slider = new Swiper ('.swiper-container', {
        // Optional parameters
        direction: 'horizontal',
        loop: true,
        slidesPerView: 1,
        centeredSlides: true,
        observer: true,
        observeParents: true,

        // If we need pagination
        pagination: {
          el: '.slider-pagination',
          type: 'fraction',
        },

        // Navigation arrows
        navigation: {
          nextEl: '.slider-next',
          prevEl: '.slider-prev',
        },

        on: {
          slideChangeTransitionStart: function () {
              // Slide captions
              var swiper = this;
              var slideTitle = $(swiper.slides[swiper.activeIndex]).attr("data-caption");
              $(".slide-captions").html(function() {
                return "<p class='current-title'>" + slideTitle + "</p>";
              });
          }
        }

    });

    // Slide captions on load
    var sizes1 = $(slider.slides[slider.activeIndex]).attr("data-caption");
    	$(".slide-captions").html(function() {
        return "<p class='current-title'>" + sizes1 + "</p>";
    });

[nothinghalosix]

same issue here