Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 35354)
+++ wp-includes/media.php	(working copy)
@@ -2909,7 +2909,7 @@
 
 			/** This filter is documented in wp-includes/media.php */
 			if ( $downsize = apply_filters( 'image_downsize', false, $attachment->ID, $size ) ) {
-				if ( ! $downsize[3] )
+				if ( ! isset( $downsize[3] ) )
 					continue;
 				$sizes[ $size ] = array(
 					'height'      => $downsize[2],