Index: src/wp-content/themes/twentyfifteen/inc/template-tags.php
===================================================================
--- src/wp-content/themes/twentyfifteen/inc/template-tags.php	(revision 48950)
+++ src/wp-content/themes/twentyfifteen/inc/template-tags.php	(working copy)
@@ -121,8 +121,8 @@
 				'<span class="full-size-link"><span class="screen-reader-text">%1$s </span><a href="%2$s">%3$s &times; %4$s</a></span>',
 				_x( 'Full size', 'Used before full size attachment link.', 'twentyfifteen' ),
 				esc_url( wp_get_attachment_url() ),
-				$metadata['width'],
-				$metadata['height']
+				isset( $metadata['width'] ) ? $metadata['width'] : 0,
+				isset( $metadata['height'] ) ? $metadata['height'] : 0
 			);
 		}