diff -Naur /wp-includes/rest-api.php /wp-includes/rest-api.php
--- /wp-includes/rest-api.php	2017-10-24 21:05:50.000000000 +0530
+++ /wp-includes/rest-api.php	2018-09-19 15:29:46.067359702 +0530
@@ -1080,7 +1080,7 @@
  * @return true|WP_Error
  */
 function rest_validate_value_from_schema( $value, $args, $param = '' ) {
-	if ( 'array' === $args['type'] ) {
+	if ( isset( $args['type'] ) && 'array' === $args['type'] ) {
 		if ( ! is_array( $value ) ) {
 			$value = preg_split( '/[\s,]+/', $value );
 		}
@@ -1221,7 +1221,7 @@
  * @return true|WP_Error
  */
 function rest_sanitize_value_from_schema( $value, $args ) {
-	if ( 'array' === $args['type'] ) {
+	if ( isset( $args['type'] ) && 'array' === $args['type'] ) {
 		if ( empty( $args['items'] ) ) {
 			return (array) $value;
 		}