Adding thumbnails to AZ index

I habe the same problem to Ashanti

I use wp plugin Az index for alphabetical catalog and want to add post thumbnails. For this I need correct thumbnail function for thumb image link. In the developer sample it works with custom field and looks as:

$thumb = get_post_custom_values(‘Thumb’, $item[‘id’]);

In my code I want to use theme’s thumbnail function:

$thumb = get_the_post_thumbnail;

Tried different variants, no result. Displays broken image (other thumb plugins work correctly).

Can somebody help me please? What is mistake?