A field with dashes in the slug

How do I reach a specific userdata in the database, when that field has a dash (hyphen) - in the slug?

I use the plugin WP-Members.

I have created a new userdata field with the plugin with the name date-of-birth – by mistake I used these dashes - instead of underscores _. In the database (the usermeta table) it now looks like:

I get the userdata with these two lines:

$user_data = get_userdata( $user_id );
$user_data->user_email;

which works for all fields – exept those with dashes in them! I have tried this:

$user_data = get_userdata( $user_id );
$user_data->date-of-birth;

which just returns 0. And this:

$user_data = get_userdata( $user_id );
$user_data->date_of_birth;

which of course doesn’t work (it gives null), since that is not the correct name.

Is there a method to reach this field with dashes in the slug?

Note
I can’t change the slug. That is not allowed by this plugin after the field is created.