Problems with exporting at 3ds max

Hey guys, I got a problem, Im trying to export a model i edited as a .mdx with 3ds max, I selected all, when I click at "exporte selected as a MDX" comes an error with the msg : "unable to convert: Undefined to type: integer". What could be wrong? plz help!!
Thx

What exporter are you using?

Im using "KMK's exporter/importer".
Is there any other program I could use??