I want to extend the product.category model and Odoo doesn't let me! 

class ProductCategory(models.Model):
    _inherit = 'product.category' 

I get the following error:

2015-07-16 11:06:27,473 16220 CRITICAL odoo_8a openerp.service.server: Failed to initialize database `odoo_8a`.
Traceback (most recent call last):
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\service\server.py", line 929, in preload_registries
registry = RegistryManager.new(dbname, update_module=update_module)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\registry.py", line 370, in new
openerp.modules.load_modules(registry._db, force_demo, status, update_module)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\loading.py", line 351, in load_modules
force, status, report, loaded_modules, update_module)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\loading.py", line 255, in load_marked_modules
loaded, processed = load_module_graph(cr, graph, progressdict, report=report, skip_modules=loaded_modules, perform_checks=perform_checks)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\loading.py", line 152, in load_module_graph
models = registry.load(cr, package)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\registry.py", line 163, in load
model = cls._build_model(self, cr)
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\models.py", line 595, in _build_model
original_module = pool[name]._original_module if name in parents else cls._module
File "C:\Users\laflammm\Desktop\GitHub\odoo\openerp\modules\registry.py", line 102, in __getitem__
return self.models[model_name]
KeyError: 'product.category'

I don't get it... I was able to extend res.partner and res.users with no problem! I'm using Odoo V8

Any help would be appreciated.