diff --git a/src/wp-includes/rest-api.php b/src/wp-includes/rest-api.php
index 7602dd7405..5ec557547c 100644
--- a/src/wp-includes/rest-api.php
+++ b/src/wp-includes/rest-api.php
@@ -373,7 +373,7 @@ function get_rest_url( $blog_id = null, $path = '/', $scheme = 'rest' ) {
 		$url = add_query_arg( 'rest_route', $path, $url );
 	}
 
-	if ( is_ssl() ) {
+	if ( is_ssl() && isset( $_SERVER['SERVER_NAME'] ) ) {
 		// If the current host is the same as the REST URL host, force the REST URL scheme to HTTPS.
 		if ( $_SERVER['SERVER_NAME'] === parse_url( get_home_url( $blog_id ), PHP_URL_HOST ) ) {
 			$url = set_url_scheme( $url, 'https' );