Retrieve data from database

I’ve this code:
$first_name = get_user_meta($user_id, ‘first_name’, true);
$last_name = get_user_meta($user_id, ‘last_name’, true);
$lidnummer = get_user_meta($user_id, ‘lidnummer’, true);

It’s not a problem to retrieve the first_name and last_name but it’s impossible to retrieve the data from lidnummer. The result is always empty.
Lidnummer is a custom field with type “Input” and Meta-name “lidnummer”.

When I check the table there is a value for lidnummer present. I don’t see what’s wrong.
Do I have to call a custom field different than a standard field? Something else?