Error in media library with upgrade to 2.7

When writing a post and click on add media – then go to add from existing media library – get following error:

Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /home/pdance/public_html/blog/wp-admin/includes/media.php on line 736

This issue has only occurred after the upgrade. No functionality appears to have been lost and can still use images.

Also note from dashboard, clicking to view media library does NOT generate any errors.

Any ideas?