What's wrong with my Python code for a vanity plate checker?

Question

Alright, I'm stuck. Should be a simple Vanity Plate code, but for some reason its only giving me Invalids, tried to cheat the autograder system and its still not working with me. What am I missing?

def main():
    # Prompt the user for a vanity plate
    plate = input("Plate: ")
    if is_valid(plate):
        print("Valid")
    else:
        print("Invalid")

def is_valid(s):
    # Check if the plate meets all reqs
    return (
        has_valid_length(s) and
        has_valid_format(s) and
        has_valid_characters(s)
    )

def has_valid_length(s):
    # Check if the plate has a length of 6 char
    return len(s) == 6

def has_valid_characters(s):
    # Check if the plate  only has alphanumeric characters
    return s.isalnum()

def has_valid_format(s):
    # Check if the plate matches the specific formats "CS50" or "ECTO88",
    # or if it has two characters followed by digits
    return s in ["CS50", "ECTO88"] or (s[:2].isalpha() and s[2:].isdigit())

main()

Tried to change the retun, restructed my methods, tried to just exclude the specific text and still no go.