Bug Practice: Add One Function (submission has checkstyle errors)

When I implement the exercise “AddOne Function” in the chapter of Functions, my correct submission is not accepted (the same as the walkthrough) and the error message I receive is:
Your submission has checkstyle errors:
Question.java:2: checkstyle error: Assignment of parameter ‘val’ is not allowed.
1 error

When I have a line “val++” I receive that error, but that’s one of the same solution shown in the walkthrough.

Do you have seen this kind of error before?

PS: so far the content is very great! Thanks for this platform.