What is class_ for a dictionary of lists of strings?

I’m working on some code that uses Python’s param package to initialize variables:

import param

class UnivariateConfig(param.Parameterized):
    input_directory = param.String(default='')
    output_directory = param.String(default='outputs')
    univariate_config = param.String(default='')
    time_types = param.List(default=[], class_=str)
    questionnaire_types = param.List(default=[], class_=str)
    label_types = param.List(default=[], class_=str)

I’m struggling with the param documentation, I cannot find what the values of class_ should be for a dictionary I would normally type as

model_types: Dict[str, List[str]] = {}

How would I rewrite that line to use param.Dict, i.e. what should I write in place of WhichClass in this:

model_types = param.Dict(default={}, class_=WhichClass)