diff -pur a/wp-includes/media.php b/wp-includes/media.php
--- a/wp-includes/media.php	2020-11-19 21:25:28.051455651 +0100
+++ b/wp-includes/media.php	2020-11-19 21:24:42.735165966 +0100
@@ -2386,6 +2386,11 @@ function gallery_shortcode( $attr ) {
 	}
 
 	if ( ! $html5 && $columns > 0 && 0 !== $i % $columns ) {
+		while ( 0 !== $i % $columns ) {
+			$output .= "<{$itemtag} class='gallery-item'>";
+			$output .= "</{$itemtag}>";
+			$i++;
+		}
 		$output .= "
 			<br style='clear: both' />";
 	}