diff --git src/wp-includes/class-wp-customize-manager.php src/wp-includes/class-wp-customize-manager.php
index f80708a23f..ec27563b61 100644
--- src/wp-includes/class-wp-customize-manager.php
+++ src/wp-includes/class-wp-customize-manager.php
@@ -997,13 +997,19 @@ final class WP_Customize_Manager {
 			wp_list_pluck( $posts, 'post_name' )
 		);
 
+		/*
+		 * Obtain all post types referenced in starter content to use in query.
+		 * This is needed because 'any' will not account for post types not yet registered.
+		 */
+		$post_types = array_filter( array_merge( array( 'attachment' ), wp_list_pluck( $posts, 'post_type' ) ) );
+
 		// Re-use auto-draft starter content posts referenced in the current customized state.
 		$existing_starter_content_posts = array();
 		if ( ! empty( $starter_content_auto_draft_post_ids ) ) {
 			$existing_posts_query = new WP_Query( array(
 				'post__in' => $starter_content_auto_draft_post_ids,
 				'post_status' => 'auto-draft',
-				'post_type' => 'any',
+				'post_type' => $post_types,
 				'posts_per_page' => -1,
 			) );
 			foreach ( $existing_posts_query->posts as $existing_post ) {
diff --git src/wp-includes/class-wp-customize-nav-menus.php src/wp-includes/class-wp-customize-nav-menus.php
index 28e8196917..998270271d 100644
--- src/wp-includes/class-wp-customize-nav-menus.php
+++ src/wp-includes/class-wp-customize-nav-menus.php
@@ -786,7 +786,7 @@ final class WP_Customize_Nav_Menus {
 	 * @return WP_Post|WP_Error Inserted auto-draft post object or error.
 	 */
 	public function insert_auto_draft_post( $postarr ) {
-		if ( ! isset( $postarr['post_type'] ) || ! post_type_exists( $postarr['post_type'] )  ) {
+		if ( ! isset( $postarr['post_type'] ) ) {
 			return new WP_Error( 'unknown_post_type', __( 'Invalid post type.' ) );
 		}
 		if ( empty( $postarr['post_title'] ) ) {
diff --git tests/phpunit/tests/customize/manager.php tests/phpunit/tests/customize/manager.php
index 033fa49731..4590772ddc 100644
--- tests/phpunit/tests/customize/manager.php
+++ tests/phpunit/tests/customize/manager.php
@@ -373,6 +373,10 @@ class Tests_WP_Customize_Manager extends WP_UnitTestCase {
 					'post_title' => 'Custom',
 					'thumbnail' => '{{waffles}}',
 				),
+				'unknown_cpt' => array(
+					'post_type' => 'unknown_cpt',
+					'post_title' => 'Unknown CPT',
+				),
 			),
 			'attachments' => array(
 				'waffles' => array(
@@ -441,7 +445,7 @@ class Tests_WP_Customize_Manager extends WP_UnitTestCase {
 		$this->assertEquals( array( 'text-2', 'meta-3' ), $changeset_values['sidebars_widgets[sidebar-1]'] );
 
 		$posts_by_name = array();
-		$this->assertCount( 6, $changeset_values['nav_menus_created_posts'] );
+		$this->assertCount( 7, $changeset_values['nav_menus_created_posts'] );
 		$this->assertContains( $existing_published_home_page_id, $changeset_values['nav_menus_created_posts'], 'Expected reuse of non-auto-draft posts.' );
 		$this->assertContains( $existing_canola_attachment_id, $changeset_values['nav_menus_created_posts'], 'Expected reuse of non-auto-draft attachment.' );
 		$this->assertNotContains( $existing_auto_draft_about_page_id, $changeset_values['nav_menus_created_posts'], 'Expected non-reuse of auto-draft posts.' );
@@ -461,7 +465,7 @@ class Tests_WP_Customize_Manager extends WP_UnitTestCase {
 			}
 			$posts_by_name[ $post_name ] = $post->ID;
 		}
-		$this->assertEquals( array( 'waffles', 'canola', 'home', 'about', 'blog', 'custom' ), array_keys( $posts_by_name ) );
+		$this->assertEquals( array( 'waffles', 'canola', 'home', 'about', 'blog', 'custom', 'unknown-cpt' ), array_keys( $posts_by_name ) );
 		$this->assertEquals( 'Custom', get_post( $posts_by_name['custom'] )->post_title );
 		$this->assertEquals( 'sample-page-template.php', get_page_template_slug( $posts_by_name['about'] ) );
 		$this->assertEquals( '', get_page_template_slug( $posts_by_name['blog'] ) );
diff --git tests/phpunit/tests/customize/nav-menus.php tests/phpunit/tests/customize/nav-menus.php
index e68ab0539a..77116305e2 100644
--- tests/phpunit/tests/customize/nav-menus.php
+++ tests/phpunit/tests/customize/nav-menus.php
@@ -538,9 +538,13 @@ class Test_WP_Customize_Nav_Menus extends WP_UnitTestCase {
 		$this->assertInstanceOf( 'WP_Error', $r );
 		$this->assertEquals( 'unknown_post_type', $r->get_error_code() );
 
-		$r = $menus->insert_auto_draft_post( array( 'post_type' => 'fake' ) );
+		// Non-existent post types allowed as of #39610.
+		$r = $menus->insert_auto_draft_post( array( 'post_title' => 'Non-existent', 'post_type' => 'nonexistent' ) );
+		$this->assertInstanceOf( 'WP_Post', $r );
+
+		$r = $menus->insert_auto_draft_post( array( 'post_type' => 'post' ) );
 		$this->assertInstanceOf( 'WP_Error', $r );
-		$this->assertEquals( 'unknown_post_type', $r->get_error_code() );
+		$this->assertEquals( 'empty_title', $r->get_error_code() );
 
 		$r = $menus->insert_auto_draft_post( array( 'post_status' => 'publish', 'post_title' => 'Bad', 'post_type' => 'post' ) );
 		$this->assertInstanceOf( 'WP_Error', $r );