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formal series over InfinitePolynomialRing

I apologize if the question does not belong here. This is my first try to using sage and I find the documentation hard to read/search. I am trying to work with symbolic power series over a non-Noetherian ring. So for example I have:

sage: P.<x> = InfinitePolynomialRing(QQ) 
sage: R.<t> = PowerSeriesRing(P)

And I'd like to consider the series $f(t) = \sum x_n t^n$ as an element of R. But my first try


    sage: sum(x[n]*t^n,(n,0,oo))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-27-d7d20df9d062> in <module>()
----> 1 sum(x[n]*t**n,(n,Integer(0),oo))

/usr/lib64/python2.7/site-packages/sage/rings/polynomial/infinite_polynomial_ring.pyc in __getitem__(self, i)
   1433             alpha_1
   1434         """
-> 1435         if int(i) != i:
   1436             raise ValueError("The index (= %s) must be an integer" % i)
   1437         i = int(i)

TypeError: int() argument must be a string or a number, not 'function'

I'll appreciate any help or if you can point me to the documentation where to read about this.