protected function getAttachmentFallbackChain( $user_id ) {
		$chain = [];

		foreach ( $this->avatar_user_meta_keys as $user_meta_key ) {
			$attachment_id = get_user_meta( $user_id, $user_meta_key, true );
			if ( is_numeric( $attachment_id ) ) {
				$chain[] = intval( $attachment_id );
			}
		}

		if ( $this->default_avatar_id !== 0 ) {
			$chain[] = $this->default_avatar_id;
		}

		return $chain;
	}