oracle procedure ussing Host() command

oracle procedure ussing Host() command

I am having a problem using host() command in oracle procedure. I have
written very simple oracle code.
CREATE OR REPLACE PROCEDURE
run_command(command_i IN VARCHAR2)
IS l_message VARCHAR2 (100);
BEGIN
l_message := 'cmd ' || command_i;
host(l_message);
END run_command;
when host(l_message); is eliminated works fine..
Whats the problem and is there anyway to create a routine which usses
host()??