diff --git a/lib/mastodon.php b/lib/mastodon.php
index bb4c7dd..d5fda7a 100644
--- a/lib/mastodon.php
+++ b/lib/mastodon.php
@@ -5,6 +5,16 @@
 // require gettlds.php in the calling script and use it to set it like this:
 // $retlds=gettlds($path_to_tlds_cache_file,true);
 
+/*
+Warning: postlength function requires $retlds global variable to be defined.
+$retlds has to be a string with a list (better if reverse ordered) of valid
+tlds separated by "|"; you can get such a list by adding "require gettlds.php"
+in the calling script like this:
+$retlds=gettlds($path_to_tlds_cache_file,true);
+$retlds=$retlds['tlds'];
+($retlds['warnings'] will contain possible gettlds warnings)
+*/
+
 function validtoken($token) {
 	if (preg_match('#^[A-Za-z0-9_-]{43}$#',$token)===1)
 		return true;