[dba-Tech] MySQL Syntax: Fresh eyes needed

I've been staring at this expression too long and my eyes are beginning to
cross:
<,mysql>
SELECT
IF(ISNULL(customers.`Last Name`),customers.Company,CONCAT(customers.'First
Name',' ', customers.'Last Name')) as 'File As:'
from `customers`
</mysql>

Syntax error. The Concat() part works fine in isolation so the problem is
not there

TIA,
Arthur