wp-menu -> list doesn't get an id

Hi there

I’m just about to build my first wordpresstheme and I’ve already encountered my first problems.
one of them is, that although i’ve set this code in the header.php

<?php wp_nav_menu(array(‘container’=>’ul’,’menu_id’=>’nav’)); ?>

my list (ul) doesn’t get the id “nav”. I’ve tested the page in safaris web developer tool and I set the id of the ul manually. then it works all fine. So the point for the uncorrect appearance is really found here.

Can somebody tell me where to find the mistake?
I’ve double checked this php-code and can’t find any wrong detail….

Thx already