Any suggestions with array_merge error

PHP Warning:  array_merge() [<a href='function.array-merge'>function.array-merge</a> ]: Argument #1 is not an array in /home/public_html/inc/classes/BxDolMemberMenu.php on line 517

Comes up when a user logs into account.

any suggestions on how to fix?

Hello, check permission for cache/ (must be writable)