Index: src/wp-includes/rest-api/class-wp-rest-server.php
===================================================================
--- src/wp-includes/rest-api/class-wp-rest-server.php	(revision 39885)
+++ src/wp-includes/rest-api/class-wp-rest-server.php	(working copy)
@@ -291,6 +291,7 @@
 		}
 
 		if ( empty( $path ) ) {
+			var_dump( $path );
 			if ( isset( $_SERVER['PATH_INFO'] ) ) {
 				$path = $_SERVER['PATH_INFO'];
 			} else {
Index: src/wp-includes/rest-api.php
===================================================================
--- src/wp-includes/rest-api.php	(revision 39885)
+++ src/wp-includes/rest-api.php	(working copy)
@@ -256,7 +256,11 @@
 	$server = rest_get_server();
 
 	// Fire off the request.
-	$server->serve_request( untrailingslashit( $GLOBALS['wp']->query_vars['rest_route'] ) );
+	$route = untrailingslashit( $GLOBALS['wp']->query_vars['rest_route'] );
+	if ( empty( $route ) ) {
+		$route = '/';
+	}
+	$server->serve_request( $route );
 
 	// We're done.
 	die();