Index: src/wp-includes/post.php
===================================================================
--- src/wp-includes/post.php	(リビジョン 43909)
+++ src/wp-includes/post.php	(作業コピー)
@@ -4546,7 +4546,7 @@
 		$excerpt = strip_tags( $postdata['post_excerpt'] ? $postdata['post_excerpt'] : $postdata['post_content'] );
 
 		if ( strlen( $excerpt ) > 255 ) {
-			$excerpt = substr( $excerpt, 0, 252 ) . '…';
+			$excerpt = mb_strcut( $excerpt, 0, 252 ) . '…';
 		}
 
 		$trackback_urls = explode( ',', $tb_list );