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Obtaining a poset from Posets(n)

I have defined function that takes a poset $P$ as input and outputs the major index generating polynomial over all linear extensions of $P$. Here, the major index of a certain linear extension of $P$ is given by the major index of the corresponding permutation, see e.g. for an example of this correspondence.

def major_index(pos):
    poly = 0
    lst = pos.linear_extensions()
    for le in lst: 
        perm = Permutation(x.element for x in le)
        maj = perm.major_index()
        poly += x^maj
    return poly

If I write, for example

P = Poset(([1,2,3,4], [[1,3],[1,4],[2,3]]), linear_extension=True, facade=False)

Then I get

x^4 + x^3 + x^2 + x + 1

which is the correct output. I would like to use Posets(n) to generate all posets (up to isomorphism) with $n$ vertices. However, when I write

lst = Posets(3)
P = lst[3]

I get the error:

Traceback (click to the left of this block for traceback)
AttributeError: 'sage.rings.integer.Integer' object has no attribute

Clearly the error is due to the line

perm = Permutation(x.element for x in le)

But I don't understand how Posets(n) works and why I cannot just write lst[3] to obtain a poset.