Context.
I have setup products and variants for purchases. When I'm creating a new purchase order, I enter a product and the popup window to select a variant is displayed. I select the variant and enter the quantity. When I clic ok, the error message below appears. I have duplicated the error in runbot and it happens exctly as it happens on my environment. I'm not a developer, so I have a limited understanding of code. I have searched for 
product_custom_attribute_value_ids in purchase.order.line and I can't find it. None of the attritbutes is a custom value. Help!
Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/models.py", line 5354, in _update_cache
    field_values = [(fields[name], value) for name, value in values.items()]
  File "/odoo/odoo-server/odoo/models.py", line 5354, in 
    field_values = [(fields[name], value) for name, value in values.items()]
KeyError: 'product_custom_attribute_value_ids'
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/addons/base/models/ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "/odoo/odoo-server/odoo/http.py", line 687, in dispatch
    result = self._call_function(**self.params)
  File "/odoo/odoo-server/odoo/http.py", line 359, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/odoo/odoo-server/odoo/service/model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "/odoo/odoo-server/odoo/http.py", line 348, in checked_call
    result = self.endpoint(*a, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 916, in __call__
    return self.method(*args, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 535, in response_wrap
    response = f(*args, **kw)
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 1347, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 1339, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 464, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 451, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/odoo/odoo-server/odoo/models.py", line 6424, in onchange
    record = self.new(initial_values, origin=self)
  File "/odoo/odoo-server/odoo/models.py", line 5714, in new
    record._update_cache(values, validate=False)
  File "/odoo/odoo-server/odoo/models.py", line 5356, in _update_cache
    raise ValueError("Invalid field %r on model %r" % (e.args[0], self._name))
Exception
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/http.py", line 643, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/odoo/odoo-server/odoo/http.py", line 301, in _handle_exception
    raise exception.with_traceback(None) from new_cause
ValueError: Invalid field 'product_custom_attribute_value_ids' on model 'purchase.order.line'
