Construction challenge, not sure what I'm getting wrong. Getting a "Bummer: Didn't get the right pattern" message

class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern

    def __iter__(self):
        yield from self.pattern

    def __str__(self):
        output = []
        for blip in self:
            if blip == '.':
                output.append('dot')
            else:
                output.append('dash')
        return '-'.join(output)

    @classmethod
    def from_string(cls,string):
        pattern = []
        string = string.split("-")
        for item in string:
            if item == "dash":
                pattern.append("-")
            if item == "dot":
                pattern.append(".")
        return cls(pattern)    






class S(Letter):
    def __init__(self):
         pattern = ['.', '.', '.']
         super().__init__(pattern)