diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 2f7dc1fca7..dee0af6817 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1545,10 +1545,8 @@ function add_theme_support( $feature ) {
 			 * 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 );
+			if ( is_array( $args[0] ) && isset( $_wp_theme_features['post-thumbnails'] ) ) {
+				$args[0] = array_unique( array_merge( $_wp_theme_features['post-thumbnails'][0], $args[0] ) );
 			}
 
 			break;
diff --git a/tests/phpunit/tests/theme/support.php b/tests/phpunit/tests/theme/support.php
index 9204aee6d1..648f031797 100644
--- a/tests/phpunit/tests/theme/support.php
+++ b/tests/phpunit/tests/theme/support.php
@@ -44,8 +44,9 @@ class Tests_Theme_Support extends WP_UnitTestCase {
 	}
 
 	public function test_post_thumbnails_flat_array_of_post_types() {
+		remove_theme_support( 'post-thumbnails' );
+
 		add_theme_support( 'post-thumbnails', array( 'post', 'page' ) );
-		$this->assertTrue( current_theme_supports( 'post-thumbnails' ) );
 		$this->assertTrue( current_theme_supports( 'post-thumbnails', 'post' ) );
 		$this->assertFalse( current_theme_supports( 'post-thumbnails', 'book' ) );
 		remove_theme_support( 'post-thumbnails' );
@@ -67,6 +68,10 @@ class Tests_Theme_Support extends WP_UnitTestCase {
 
 		add_theme_support( 'post-thumbnails' );
 		$this->assertTrue( current_theme_supports( 'post-thumbnails', 'book' ) );
+
+		// Reset post-thumbnails theme support.
+		remove_theme_support( 'post-thumbnails' );
+		$this->assertFalse( current_theme_supports( 'post-thumbnails' ) );
 	}
 
 	public function test_post_thumbnails_types_true() {