Blockage doesn't work with WeightedSum and PropagateUpDownIterative

The following code crashes with UnboundLocalError: cannot access local variable 'blockage' where it is not associated with a value

from py_wake.examples.data.dtu10mw import DTU10MW
from py_wake.wind_farm_models import PropagateUpDownIterative
from py_wake.site.xrsite import UniformSite
from py_wake.deficit_models.gaussian import BastankhahGaussianDeficit
from py_wake.superposition_models import WeightedSum
from py_wake.deficit_models.selfsimilarity import SelfSimilarityDeficit2020


wfm = PropagateUpDownIterative(
    site=UniformSite(ws=8.0, ti=0.0),
    windTurbines=DTU10MW(),
    wake_deficitModel=BastankhahGaussianDeficit(use_effective_ws=True),
    superpositionModel=WeightedSum(),
    blockage_deficitModel=SelfSimilarityDeficit2020(use_effective_ws=True),
)

sim_res = wfm(x=[0.0, 800.0], y=[0.0, 0.0])

The error happens here.