Get constructor argument error when bigger problem is duplicate constructor cases

[ghost]

type Msg
    = A String
    | B

update msg model =
    case msg of
        B str -> ...
        B -> ...

With the above code, I got this error:

-- TOO MANY ARGS ------------------------------------------------------ Main.elm

The `B` constructor needs 0 arguments, but I see 1 instead:

         B str ->
         ^^^^^
Which is the extra one? Maybe some parentheses are missing?

The bigger problem is that I've got two B cases in the case expression.