Error encountered when only input one perturbation

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Hi, I am trying to use Prophet following your tutorial. In your tutorial, there is a code line:

df = model.predict( target_ivs=iv_list, target_cls=cl_list, target_phs=ph_list, iv_col=['iv1', 'iv2'], num_iterations=1, save=False, )

which takes a combination of two perturbations (i.e., 'iv1' and 'iv2') as input. However, my dataset only uses one perturbation for each experiment. When I tried to run the model using only one perturbation column as the following code:

df = model.predict( target_ivs=iv_list, target_cls=cl_list, target_phs=ph_list, iv_col=['iv1'], num_iterations=1, save=False, )

I would encounter an error message below:
ValueError: Are you sure you passed the right number of intervention columns? Currently receiving ['iv1'].

How to use Prophet for prediction when I only have one perturbation for each experiment?