[Exim] Problems with exim.conf

I've just installed exim 3.12 with mysql support, but when I add the
line:

hide mysql_servers = localhost/sourceforge/sfexim/p1k@chU

to the /etc/exim.conf file and try to run the exim program in the
command line, it gives the following error:

main option "hide" unknown in line 46

When I don't put the 'hide' everything goes OK. What's wrong?