Retrieving Image Caption Doesn't Seem to Be Working

I can’t seem to retrieve the caption of images. Here’s my current code, minus my echo/output statements:

if( class_exists('Dynamic_Featured_Image') ) {
     global $dynamic_featured_image;
     $featured_images = $dynamic_featured_image->get_featured_images();

      foreach($featured_images as $image) {
	    $url = $image['full'];
	    $caption = $dynamic_featured_image->get_image_caption($url);
	}
}

$url returns the correct image URL, but $caption consistently comes back blank, even when directly passed a hardcoded (valid) string value for an image URL.

Any ideas? Thanks in advance.

https://wordpress.org/plugins/dynamic-featured-image/