Cannot use string offset as an array

[rscape]

Whenever I run this code:

if (($_GET["premium_acc"] == "on" && $_GET["premium_user"] && $_GET["premium_pass"]) || ($_GET["premium_acc"] == "on" && $premium_acc["rs_com"]["user"] && $premium_acc["rs_com"]["pass"])) 

I get the error:

Fatal error: Cannot use string offset as an array in com.php on line 8

Is there anything wrong in the code?