Index: src/wp-includes/class-wp-customize-nav-menus.php
===================================================================
--- src/wp-includes/class-wp-customize-nav-menus.php	(revision 41185)
+++ src/wp-includes/class-wp-customize-nav-menus.php	(working copy)
@@ -1210,7 +1210,7 @@
 			! empty( $args['echo'] )
 			&&
 			// ...and if the fallback_cb can be serialized to JSON, since it will be included in the placement context data,
-			( empty( $args['fallback_cb'] ) || is_string( $args['fallback_cb'] ) )
+			( empty( $args['fallback_cb'] ) || ( is_string( $args['fallback_cb'] ) && false === strpos( $args['fallback_cb'], '\\' ) ) )
 			&&
 			// ...and if the walker can also be serialized to JSON, since it will be included in the placement context data as well,
 			( empty( $args['walker'] ) || is_string( $args['walker'] ) )