I am building a simplicial complex as follows:

sage:S=range(1,7) sage:Z=SimplicialComplex([S]) sage:T=Z.n_skeleton(1) sage:T.faces() {0: set([(4,), (5,), (3,), (1,), (6,), (2,)]), 1: set([(2, 4), (3, 6), (5, 6), (2, 6), (1, 4), (3, 5), (3, 4), (2, 3), (4, 6), (2, 5), (1, 2), (1, 3), (4, 5), (1, 5), (1, 6)]), -1: set([()])} sage:T.homology() {0: 0, 1: Z^10}

So far everything seems good. Then I try adding a face to T.
sage:T.add_face([1,2,6])
sage:T.faces()

{0: set([(4,), (5,), (3,), (1,), (6,), (2,)]), 1: set([(2, 4), (3, 6),
(5, 6), (1, 5), (1, 4), (3, 5), (3, 4), (2, 3), (4, 6), (2, 5), (1, 2),
(1, 3), (1, 6), (2, 6), (4, 5)]), 2: set([(1, 2, 6)]), -1: set([()])}

So the face seems to have been added.

But then:

sage:T.homology()

results in:

{0: 0, 1: Z^10, 2: 0}

But this doesn't make any sense --- clearly it should say 0:0, 1:Z^9, 2:0, since adding a 2-face kills a class in $H_1(T)$.

Can anyone tell what I'm doing wrong?