Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 16210)
+++ wp-includes/post-template.php	(working copy)
@@ -327,8 +327,12 @@
 	$classes[] = $post->post_type;
 	$classes[] = 'type-' . $post->post_type;
 	$classes[] = 'status-' . $post->post_status;
-	if ( get_post_format( $post->ID ) )
-		$classes[] = 'format-' . sanitize_html_class( get_post_format( $post->ID ) );
+	
+	// Post Format
+	$post_format = get_post_format( $post->ID );
+
+	if ( $post_format && !is_wp_error($post_format) )
+		$classes[] = 'format-' . sanitize_html_class( $post_format );
 	else
 		$classes[] = 'format-default';
 
@@ -424,8 +428,12 @@
 		$classes[] = 'single';
 		$classes[] = 'single-' . sanitize_html_class($post->post_type, $post_id);
 		$classes[] = 'postid-' . $post_id;
-		if ( get_post_format( $post_id ) )
-			$classes[] = 'single-format-' . sanitize_html_class( get_post_format( $post_id ) );
+		
+		// Post Format
+		$post_format = get_post_format( $post->ID );
+
+		if ( $post_format && !is_wp_error($post_format) )
+			$classes[] = 'single-format-' . sanitize_html_class( $post_format );
 		else
 			$classes[] = 'single-format-default';
 
Index: wp-admin/includes/class-wp-posts-list-table.php
===================================================================
--- wp-admin/includes/class-wp-posts-list-table.php	(revision 16210)
+++ wp-admin/includes/class-wp-posts-list-table.php	(working copy)
@@ -468,8 +468,10 @@
 		$title = _draft_or_post_title();
 		$post_type_object = get_post_type_object( $post->post_type );
 		$can_edit_post = current_user_can( 'edit_post', $post->ID );
+		$post_format = get_post_format( $post->ID );
+		$post_format_class = ( $post_format && !is_wp_error($post_format) ) ? 'format-' . sanitize_html_class( $post_format ) : 'format-default';
 	?>
-		<tr id='post-<?php echo $post->ID; ?>' class='<?php echo trim( $rowclass . ' author-' . $post_owner . ' status-' . $post->post_status . ' ' . sanitize_html_class( 'format-' . ( get_post_format( $post->ID ) ? get_post_format( $post->ID ) : 'default' ) ) ); ?> iedit' valign="top">
+		<tr id='post-<?php echo $post->ID; ?>' class='<?php echo trim( $rowclass . ' author-' . $post_owner . ' status-' . $post->post_status . ' ' . $post_format_class); ?> iedit' valign="top">
 	<?php
 
 		list( $columns, $hidden ) = $this->get_column_info();