[rhn-users] unary operator expected ??

[FM]

Hello my crontab is :

if [ `ls /home/bc_bigfiles/pub` ]; then echo "Files found in server 
`hostname`"; fi ;

in the folder pub :
NBCA_20050418_1.zip  NBQB_20050418_2.zip

Now I received ths error :
NBCA_20050418_1.zip: unary operator expected

What does iy mean ? On the web I found some pages talking about unset 
variable, which it is not my case, I have none :-)

Thanks !