Morse code task might have an error?

I keep getting this error message, which seems to be the output it wants?

Bummer: Didn't get the right string output. Got: dot-dot-dot for S()

here is the original question:

Challenge Task 1 of 1 Let's use str to turn Python code into Morse code! OK, not really, but we can turn class instances into a representation of their Morse code counterparts.

I want you to add a str method to the Letter class that loops through the pattern attribute of an instance and returns "dot" for every "." (period) and "dash" for every "_" (underscore). Join them with a hyphen.

I've included an S class as an example (I'll generate the others when I test your code) and it's str output should be "dot-dot-dot".

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


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

    def __str__(self):
        morse_text = []
        for item in self.pattern:
            if item == ".":
                morse_text.append("dot")
            elif item == "_":
                morse_text.append("dash")
        morse_text = "-".join(morse_text)
        return morse_text