Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 23088)
+++ wp-admin/includes/post.php	(working copy)
@@ -235,8 +235,8 @@
 			}
 		}
 
-		if ( isset( $post_data['attachments'][ $post_ID ] ) )
-			$post_data = apply_filters( 'attachment_fields_to_save', $post_data, $post_data['attachments'][ $post_ID ] );
+		$attachment_data = isset( $post_data['attachments'][ $post_ID ] ) ? $post_data['attachments'][ $post_ID ] : array();
+		$post_data = apply_filters( 'attachment_fields_to_save', $post_data, $attachment_data );
 	}
 
 	add_meta( $post_ID );