nearest-value checking error

I would like to give some feedback about ...

From: https://py.checkio.org/mission/nearest-value/solve/

HTTP_USER_AGENT: The question provided the variable should be set as `def nearest_value(values: set, one: int)`, however, I provided my answer that run correct, by clicking Check it shows AttributeError: 'list' object has no attribute 'add'.<br> I believe there should bu something wrong here.

def nearest_value(values: set, one: int) -> int:
    if one in values:
        return one
    else:
        values.add(one)
        a = list(values)
        a.sort()
        p = a.index(one)
        if p == len(a)-1:
            return a[p-1]
        else:
            return a[p+1] if one-a[p-1] > a[p+1]-one or p==0 else a[p-1]

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