Voids will be converted to strings only as they come with generic or specific messages such ase "# Invalid Value #". The same conversion method applies as for strings. See String to String Formatting Parameters for details.
echo( str( null(), "20x", align right, '!' ) );
echo( str( null(), "X X X X X X X X X X X X X X X X X X X X" ) );
!!!# invalid value #
# I N V A L I D V A L U E #