Tensor product of polynomial algebras

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I want to make the tensor product of polynomial algebras. Evidently, the following is not the right way to do this:

sage: P.<x> = PolynomialRing(QQ)
sage: tensor([P, P])
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-2-17decd317813> in <module>()
----> 1 tensor([P, P])

/usr/lib/python2.7/site-packages/sage/categories/covariant_functorial_construction.pyc in __call__(self, args, **kwargs)
    219         """
    220         args = tuple(args) # a bit brute force; let's see if this becomes a bottleneck later
--> 221         assert(all( hasattr(arg, self._functor_name) for arg in args))
    222         assert(len(args) > 0)
    223         return getattr(args[0], self._functor_name)(*args[1:], **kwargs)

AssertionError:

This does work for CombinatorialFreeModules but I can't figure out how to turn one of those into a polynomial algebra.