Warning: implode() [function.implode]

Hi everyone,

I have a problem with my wordpress template

I have the following error message on top of the blog section of my website :

Warning: implode() [function.implode]: Invalid arguments passed in template-blog.php on line 21

The code on that page is the following :

‘paged’ => $paged, ‘cat’ => implode(“,”, $blog_categories) ) );

I am not a specialist of PHP so i really don′t know how to solve this problem.

Thank you for your help