concatenation issue ( 8.4 )

I have a table with
    name_first
    name_middle
    name_last

if i try concatenating as such:
    SELECT
        name_first || ' ' || name_middle || ' ' || name_last
    FROM
        mytable
    ;

I end up with NULL as the concatenated string whenever any of the
referred fields contain a NULL value

I tried some text conversion and explicit casting , but that didn't work

What am I doing wrong ?