MySQL trigger update error 1111 Trying to update the "size" column with the value of "file" column.

BEGIN
  set NEW.size=SUM(LENGTH(file));
END




Executing:

UPDATE `table`.`library` SET `_by`='Danis555, Vovan244' WHERE `id`='20';



  Operation failed: There was an error while applying the SQL script to the database.
  
  ERROR 1111: 1111: Invalid use of group function
  SQL Statement:
  UPDATE table.library SET _by='Danis555, Vovan244' WHERE id='20'
SET NEW.size = LENGTH(OLD.file);


No SUM, since there is only one row.
Specify which file with OLD..