Help me with WP Super Cache lugin on multi site

Hi,

I installed and network enable with WP Super Cache plugin .

In my subsite , i set up active :
+Caching On
+Use mod_rewrite to serve cache files.

And this plugin notice :
Notice: WP Super Cache mod_rewrite rule checks disabled unless running on the main site of this network.

I don't understand what that notice mean , is it normal ?

Please guide me . Thanks .