Problem with Hottbj Exporter

Hi All
I imported an .wrl object and changed the scale an axes. Now, I d’like to export this object as a .xml file with hottbj for using it in jMonkeyEngine. After I used the Export button a failure message is shown with message “vertMatIndex has a None Element”. I started Blender in command window. Here is the output:

TODO WARNING: JmeMesh should constrain ‘__unify’ to ‘3’ if the corresponding JmeNode is morphNode or MORPHGEO or SKIN
/home/imazwerg/.blender/scripts/hottbj/gui.py:127
/home/imazwerg/.blender/scripts/hottbj/exporter.py:229
/home/imazwerg/.blender/scripts/hottbj/wrapperclasses.py:2367
/home/imazwerg/.blender/scripts/hottbj/wrapperclasses.py:511
/home/imazwerg/.blender/scripts/hottbj/wrapperclasses.py:911
vertMatIndex has a None element

Does anybody has an idea, what I am doing wrong? Please help.

Best Regards MWuet