comparing a DATE column with "one minute ago", type warning?

How should I be comparing a DATE column with "one minute ago"?

I do this:

    select sysdate, sysdate- interval '1' minute from dual;

and get just what I need, the time a minute ago:

    SYSDATE                   SYSDATE-INTERVAL'1'MINUTE 
    ------------------------- ------------------------- 
    08-OCT-08 17:09:19        08-OCT-08 17:08:19        

But in my PL/SQL, I get a warning on this code, where lastping is a DATE column:

        update mytable
           set status='silent'
         where lastping < (sysdate - interval '1' minute);
                           ^       ^
                      col 28     col 36

Warning(16,28): PLW-07202: bind type would result in conversion
                away from column type
Warning(16,36): PLW-07202: bind type would result in conversion
                away from column type

Many TIA!
Mark

-- 
Mark Harrison
Pixar Animation Studios