diff --git a/multi-post-thumbnails.php b/multi-post-thumbnails.php
index cdbdf3c..898b200 100644
--- a/multi-post-thumbnails.php
+++ b/multi-post-thumbnails.php
@@ -313,9 +313,9 @@ public static function the_post_thumbnail($post_type, $thumb_id, $post_id = null
 		 * @param bool $link_to_original Optional. Wrap link to original image around thumbnail?
 		 * @param string|array $attr Optional. Query string or array of attributes.
 		  */
-		public static function get_the_post_thumbnail($post_type, $thumb_id, $post_id = NULL, $size = 'post-thumbnail', $attr = '' , $link_to_original = false) {
+		public static function get_the_post_thumbnail($post_type, $thumb_id, $post_id = null, $size = 'post-thumbnail', $attr = '' , $link_to_original = false) {
 			global $id;
-			$post_id = (NULL === $post_id) ? get_the_ID() : $post_id;
+			$post_id = (null === $post_id) ? get_the_ID() : $post_id;
 			$post_thumbnail_id = self::get_post_thumbnail_id($post_type, $thumb_id, $post_id);
 			$size = apply_filters("{$post_type}_{$post_id}_thumbnail_size", $size);
 			if ($post_thumbnail_id) {