This fuzzer will run into a Debug.crash:

myFuzz : Fuzzer Int
myFuzz =
    int |> andThen (\_ -> frequency [])

Error message reads:

    This test failed because it threw an exception: "Error: Ran into a `Debug.crash` in module `Fuzz.Internal` on line 63
    The message provided by the code author is:

        This shouldn't happen: Fuzz.Internal.unpackGenVal"

Looking at the code I don't think there's a ton we can do about this, but I wanted to report anyway. andThen, by its nature, ensures we only know whether a fuzzer is valid after running it which conflicts with the idea of fuzzer validness itself.