diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index f4dfaeedc2..2f7dc1fca7 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -1535,6 +1535,24 @@ function add_theme_support( $feature ) {
 		$args = array_slice( func_get_args(), 1 );
 
 	switch ( $feature ) {
+		case 'post-thumbnails':
+			// All post types are already supported.
+			if ( true === get_theme_support( 'post-thumbnails' ) ) {
+				return;
+			}
+
+			/*
+			 * Merge post types with any that already declared their support
+			 * for post thumbnails.
+			 */
+			if ( is_array( $args[0] ) && is_array( $_wp_theme_features['post-thumbnails'][0] ) ) {
+				$post_types = array_unique( array_merge( $args[0], $_wp_theme_features['post-thumbnails'][0] ) );
+
+				$args = array( $post_types );
+			}
+
+			break;
+
 		case 'post-formats' :
 			if ( is_array( $args[0] ) ) {
 				$post_formats = get_post_format_slugs();
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 721cf0c90b..e52c4bf0c6 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37307';
+$wp_version = '4.6-alpha-37308';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.