get_the_post_thumbnail() returning empty string on custom post type

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If nothing works, give this a try:

$img_attribs = wp_get_attachment_image_src( get_post_thumbnail_id(), 'thumbnail' ); // returns an array
if( $img_attribs ) {
?> 
<img src="https://wordpress.stackexchange.com/questions/115607/<?php echo $img_attribs[0]; ?>" width="<?php echo $img_attribs[1]; ?>" height="<?php echo $img_attribs[2]; ?>">
<?php } ?>