[Plugin: Multiple Post Thumbnails] Image cropping not working

Hi I’m using this plugin and set a new image size in the functions.php and set cropping to TRUE. However on my post page the image is not being cropped but displaying at its full size. This is when using the code for multiple image thumbnails.

Here is the code I have in functions.php

/* Add new thumbnail size */
add_image_size( 'single-post-thumbnail', 620, 250, true ); // Full image used on single post template (post.php)

/* Add secondary thumbnail (featured image) in posts */
$thumb = new MultiPostThumbnails(array(
    'label' => 'Secondary Image',
    'id' => 'secondary-image',
    'post_type' => 'post'
    )
)

and in my post.php:

if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('post', 'secondary-image')) :
	MultiPostThumbnails::the_post_thumbnail('post', 'secondary-image', NULL, 'single-post-thumnail');
endif;

How can I fix this problem? Thanks