Example notebook triggers error

In the linked example notebook it hits an error on the 4th code cell

https://mybinder.org/v2/gh/fluiddyn/fluidsim/branch%2Fdefault?urlpath=lab/tree/doc/ipynb

*************************************
Program fluidsim

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-0ebb3fef871e> in <module>
      1 params = Simul.create_default_params()
----> 2 sim = Simul(params)

/srv/conda/envs/notebook/lib/python3.6/site-packages/fluidsim/base/solvers/base.py in __init__(self, params)
    157 
    158         # complete the initialisation of the object output
--> 159         self.output.post_init()
    160 
    161         # if enabled, preprocesses flow parameters such as viscosity and

/srv/conda/envs/notebook/lib/python3.6/site-packages/fluidsim/base/output/base.py in post_init(self)
    416 class OutputBasePseudoSpectral(OutputBase):
    417     def post_init(self):
--> 418         oper = self.oper
    419         self.sum_wavenumbers = oper.sum_wavenumbers
    420         super().post_init()

AttributeError: 'Output' object has no attribute 'oper'