diff --git a/src/wp-includes/class-wp-customize-manager.php b/src/wp-includes/class-wp-customize-manager.php
index fa97e3f680..5e66a1aa75 100644
--- a/src/wp-includes/class-wp-customize-manager.php
+++ b/src/wp-includes/class-wp-customize-manager.php
@@ -271,7 +271,7 @@ final class WP_Customize_Manager {
 		}
 
 		$this->original_stylesheet = get_stylesheet();
-		$this->theme = wp_get_theme( $args['theme'] );
+		$this->theme = wp_get_theme( 0 === validate_file( $args['theme'] ) ? $args['theme'] : null );
 		$this->messenger_channel = $args['messenger_channel'];
 		$this->settings_previewed = ! empty( $args['settings_previewed'] );
 		$this->_changeset_uuid = $args['changeset_uuid'];