Support for parameters with string values

Is there a way to prevent string values from beeing converted to floats? E.g. if I define string_parameter=['a', 'b', 'c']

from doepy import build

build.full_fact({
    'int_param':[1, 2, 3],
    'float_param': [0.1, 0.2, 0.3],
    'string_param': ['a', 'b', 'c'],
)

the values for string_param are implicitly converted to 0.0, 1.0, 2.0 respectively. Instead I'd like to get the plain strings 'a', 'b', 'c'.

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