diff --git a/src/wp-includes/class-wp-oembed-controller.php b/src/wp-includes/class-wp-oembed-controller.php
index 2da7f3e5db..22c062be9f 100644
--- a/src/wp-includes/class-wp-oembed-controller.php
+++ b/src/wp-includes/class-wp-oembed-controller.php
@@ -142,12 +142,17 @@ final class WP_oEmbed_Controller {
 	 * @return string The XML response data.
 	 */
 	public function xml_response( $data ) {
+		if ( ! class_exists( 'SimpleXMLElement' ) ) {
+			status_header( 501 );
+			return get_status_header_desc( 501 );
+		}
+
 		$result = _oembed_create_xml( $data );
 
 		// Bail if there's no XML.
 		if ( ! $result ) {
 			status_header( 501 );
-			return 'Not implemented';
+			return get_status_header_desc( 501 );
 		}
 
 		if ( ! headers_sent() ) {
diff --git a/src/wp-includes/embed-functions.php b/src/wp-includes/embed-functions.php
index b9eeee0538..35e6587963 100644
--- a/src/wp-includes/embed-functions.php
+++ b/src/wp-includes/embed-functions.php
@@ -351,7 +351,10 @@ function wp_oembed_add_discovery_links() {
 
 	if ( is_singular() ) {
 		$output .= '<link rel="alternate" type="application/json+oembed" href="' . esc_url( get_oembed_endpoint_url( get_permalink() ) ) . '" />' . "\n";
-		$output .= '<link rel="alternate" type="text/xml+oembed" href="' . esc_url( get_oembed_endpoint_url( get_permalink(), 'xml' ) ) . '" />' . "\n";
+
+		if ( class_exists( 'SimpleXMLElement' ) ) {
+			$output .= '<link rel="alternate" type="text/xml+oembed" href="' . esc_url( get_oembed_endpoint_url( get_permalink(), 'xml' ) ) . '" />' . "\n";
+		}
 	}
 
 	/**
diff --git a/tests/phpunit/tests/oembed/controller.php b/tests/phpunit/tests/oembed/controller.php
index bf646f599e..34ce7dd4ee 100644
--- a/tests/phpunit/tests/oembed/controller.php
+++ b/tests/phpunit/tests/oembed/controller.php
@@ -164,9 +164,9 @@ class Test_oEmbed_Controller extends WP_UnitTestCase {
 	function test_request_xml_invalid_data() {
 		$legacy_controller = new WP_oEmbed_Controller();
 
-		$this->assertEquals( 'Not implemented',  $legacy_controller->xml_response( null ) );
-		$this->assertEquals( 'Not implemented',  $legacy_controller->xml_response( 123 ) );
-		$this->assertEquals( 'Not implemented',  $legacy_controller->xml_response( array() ) );
+		$this->assertEquals( get_status_header_desc( 501 ),  $legacy_controller->xml_response( null ) );
+		$this->assertEquals( get_status_header_desc( 501 ),  $legacy_controller->xml_response( 123 ) );
+		$this->assertEquals( get_status_header_desc( 501 ),  $legacy_controller->xml_response( array() ) );
 	}
 
 	/**