[Plugin: WordPress SEO by Yoast] Multiple vars in URL returns fatal error

I’m using a custom search page, which results in multiple get-vars in my URL. I had it all in one array (‘s’), but decided not to use it like that anymore. I changed all names in the form, test a new search and the result was a fatal error:

PHP Catchable fatal error: Object of class WP_Error could not be converted to string in /…/wp-content/plugins/wordpress-seo/frontend/class-frontend.php on line 330

Line 330 is:

echo “\t”.'<link rel=”canonical” href=”‘.$canonical.'” />’.”\n”;

And a PHP warning (just before the fatal error, got this one twice):

PHP Warning: preg_split() expects parameter 2 to be string, array given in /…/wp-includes/query.php on line 1677

I don’t have the Redirect ugly URL’s to clean permalinks. option enabled. The only Permalink option I have enabled is Enforce a trailing slash on all category and tag URL’s.

Once I deactivated the plugin the page loaded fine again.