diff --git a/src/wp-includes/post-thumbnail-template.php b/src/wp-includes/post-thumbnail-template.php
index 8202f4eb6a..85fb0e31d9 100644
--- a/src/wp-includes/post-thumbnail-template.php
+++ b/src/wp-includes/post-thumbnail-template.php
@@ -45,14 +45,14 @@ function has_post_thumbnail( $post = null ) {
  * @since 4.4.0 `$post` can be a post ID or WP_Post object.
  *
  * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
- * @return string|int Post thumbnail ID or empty string.
+ * @return int|string Post thumbnail ID or empty string if the post does not exist.
  */
 function get_post_thumbnail_id( $post = null ) {
 	$post = get_post( $post );
 	if ( ! $post ) {
 		return '';
 	}
-	return get_post_meta( $post->ID, '_thumbnail_id', true );
+	return (int) get_post_meta( $post->ID, '_thumbnail_id', true );
 }
 
 /**
diff --git a/tests/phpunit/tests/customize/manager.php b/tests/phpunit/tests/customize/manager.php
index 34fb866059..2d44bfc406 100644
--- a/tests/phpunit/tests/customize/manager.php
+++ b/tests/phpunit/tests/customize/manager.php
@@ -711,7 +711,7 @@ class Tests_WP_Customize_Manager extends WP_UnitTestCase {
 		$this->assertEquals( 'sample-page-template.php', get_page_template_slug( $posts_by_name['about'] ) );
 		$this->assertEquals( '', get_page_template_slug( $posts_by_name['blog'] ) );
 		$this->assertEquals( $posts_by_name['waffles'], get_post_thumbnail_id( $posts_by_name['custom'] ) );
-		$this->assertEquals( '', get_post_thumbnail_id( $posts_by_name['blog'] ) );
+		$this->assertEquals( 0, get_post_thumbnail_id( $posts_by_name['blog'] ) );
 		$attachment_metadata = wp_get_attachment_metadata( $posts_by_name['waffles'] );
 		$this->assertEquals( 'Waffles', get_post( $posts_by_name['waffles'] )->post_title );
 		$this->assertEquals( 'waffles', get_post_meta( $posts_by_name['waffles'], '_customize_draft_post_name', true ) );