BUG: curve failure out of bounds?

[attack68]

dates = [
     dt(2000, 1, 3),
     dt(2001, 1, 3),
     dt(2002, 1, 3),
     dt(2003, 1, 3),
     dt(2004, 1, 3),
     dt(2005, 1, 3),
     dt(2006, 1, 3),
     dt(2007, 1, 3),
     dt(2008, 1, 3),
     dt(2009, 1, 3),
     dt(2010, 1, 3),
     dt(2012, 1, 3),
     dt(2015, 1, 3),
     dt(2020, 1, 3),
     dt(2025, 1, 3),
     dt(2030, 1, 3),
     dt(2035, 1, 3),
     dt(2040, 1, 3),
     dt(2050, 1, 3),
]
curve = Curve(
    nodes={_: 1.0 for _ in dates},
    t=[dt(2000, 1, 3)] * 3 + dates[:-1] + [dt(2050, 1, 11)] * 4
)
solver = Solver(
    curves=[curve],
    instruments=[
        IRS(dt(2000, 1, 3), _, spec="gbp_irs", curves=curve) for _ in dates[1:]
    ],
    s=[1.0 + _ / 25 for _ in range(18)]
)