diff --git a/src/wp-includes/post-thumbnail-template.php b/src/wp-includes/post-thumbnail-template.php
index 3ba7450814..6c18eb3ad3 100644
--- a/src/wp-includes/post-thumbnail-template.php
+++ b/src/wp-includes/post-thumbnail-template.php
@@ -139,7 +139,8 @@ function get_the_post_thumbnail( $post = null, $size = 'post-thumbnail', $attr =
 		 *
 		 * @param int    $post_id           The post ID.
 		 * @param string $post_thumbnail_id The post thumbnail ID.
-		 * @param string $size              The post thumbnail size.
+		 * @param string $size              The post thumbnail size. Image size or array of width
+		 *                                  and height values (in that order). Default 'post-thumbnail'.
 		 */
 		do_action( 'begin_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );
 		if ( in_the_loop() )