Beta importer tool: Build failed

[Shiny Rice]

Whenever I try to preview/verify my item, a window pops up saying "Build failed" and doesn't let me go on. Here's the exact lines that appear at the end of it and cause the trouble.

d:\juegos\steam\steamapps\theinago\sourcesdk_content\tf\models\workshop\player\items\sniper\climbing_stuffings_preview5\climbing_stuffings_preview5.qc(9): - bad command <ITEMTEST_REPLACE_LOD_OPTIONALBLOCK>
ERROR: Aborted Processing on 'workshop/player/items/sniper/climbing_stuffings_preview5/climbing_stuffings_preview5.mdl'
CTargetMDL::Compile Failed - d:\juegos\steam\steamapps\common\team fortress 2\tf\models\workshop\player\items\sniper\climbing_stuffings_preview5\climbing_stuffings_preview5.mdl
WARNING: CTargetZIP::Compile - Target 3 Compile Fail

I'm importing an .fbx with 537 triangles and a dummy backpack icon to see how it looks. Texture sizes are right, 512x512. Maybe it's because they're pants, I don't really know which bones should have been assigned to the model.