Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 38455)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -1012,7 +1012,7 @@
 					$error_data = array();
 				}
 				$error_data = array_merge(
-					$error_data,
+					(array) $error_data,
 					array( 'from_server' => true )
 				);
 				$notification[ $error_code ] = array(