How to get OR pattern in Gorilla mux routing

I'm trying to use Gorilla mux router to handle paths that respond to a limited list of strings in the URL. The service I am developing will take files from the caller and pass them through an "adapter" that send them to S3 or OneDrive, depending on the "adapter" specified in the URL. I also require a variable named "schema", which I only mention now because of the weirdness that follows. My test is as follows ({schema} will be set to "test"):

router.HandleFunc("/{adapter:(s3|onedrive)}/{schema:[a-z]+}/check",
func(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte(fmt.Sprintf(`{"a":"%s","s":"%s"}`,
		mux.Vars(r)["adapter"], mux.Vars(r)["schema"])))
}).Methods("GET")

I would expect that going to /s3/test/check would yield {"a":"s3","s":"test"} just as going to /onedrive/test/check should yield {"a":"onedrive","s":"test"} ... however in these cases I am getting {"a":"s3","s":"s3"} and {"a":"onedrive","s":"onedrive"} respectively.

The (s3|onedrive) check seems to be enforced because, for example, trying to go to /dropbox/test/check correctly yields a 404.

Why is the {schema} variable getting the value of the {adapter} variable and how can I fix this?

I think it's because of parenthesis which denote capturing group and yield submatch. It can interfere with gorilla matcher. Just try without parenthesis.

router.HandleFunc("/{adapter:s3|onedrive}/{schema:[a-z]+}/check",