Notice: Trying to get property of non-object

I am getting this error on the category page (http://example.local/events/categories/my-category/) after I upgraded to WordPress 3.6.

Any suggestions of how to fix?

Notice: Undefined offset: 0 in /Users/me/WWW/example.local/wp-includes/query.php on line 2848

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3645

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3647

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3649

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3650

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3658

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/query.php on line 3673

Notice: Trying to get property of non-object in /Users/me/WWW/example.local/wp-includes/post-template.php on line 29

Thanks in advance!