FBX Export error

When i try to export my project to .fbx i get this error :

Traceback (most recent call last): File “D:\Blender\2.71\scripts\addons\io_scene_fbx_init_.py”, line 415, in execute
return export_fbx_bin.save(self, context, **keywords)
File “D:\Blender\2.71\scripts\addons\io_scene_fbx\export_fbx_bin.py”, line 2602, in save
ret = save_single(operator, context.scene, filepath, **kwargs_mod)
File “D:\Blender\2.71\scripts\addons\io_scene_fbx\export_fbx_bin.py”, line 2533, in save_single
fbx_objects_elements(root, scene_data)
File “D:\Blender\2.71\scripts\addons\io_scene_fbx\export_fbx_bin.py”, line 2384, in fbx_objects_elements
fbx_data_mesh_elements(objects, me, scene_data, done_meshes)
File “D:\Blender\2.71\scripts\addons\io_scene_fbx\export_fbx_bin.py”, line 967, in fbx_data_mesh_elements
blmats_to_fbxmats_idxs = [me_fbxmats_idx[m] for m in me_blmats]
File “D:\Blender\2.71\scripts\addons\io_scene_fbx\export_fbx_bin.py”, line 967, in <listcomp>
blmats_to_fbxmats_idxs = [me_fbxmats_idx[m] for m in me_blmats]
KeyError: None

location: <unknown location>:-1

I’ve tried a couple of things but i’m still getting this error…
EDIT : No ideas why ? I need to export my models… :frowning:

EDIT : it was actually my falt i did a wrong manipulation!