Why does clustalo guidetree have negative values for branch lengths?

Question

[O.rka]

What does it mean when clustal omega has negative values in the guidetree?

Below is a snippet:

(
Otu000011:0.0158103
,
Otu000082:0.0158103
):0
,
Otu000264:0.0158103
):0.00296443
):0.00790514
):-0.00469368
):-0.000741107
):0.000401434
):-0.000555828
):0.0146214
):-0.00712929
):-0.000969809
):-0.00141949
):0.0453094
,