fetch meta fields inside a widget

I fixed this by adding:

global $post;

More specific: I can’t explain why this works, that’s beyond my WordPress skills, but I added it right after the start of the query, so the code looks like this:

<?php global $post; 
$loop = new WP_Query( array( 
'post_type' => 'kurs', 
'posts_per_page' => '5',
'meta_key' => 'dato',
'meta_value' => date('Y-m-d'),
'meta_compare' => '>=',
'orderby' => 'meta_value',
'order' => 'ASC'
 ) ); ?>