[java / 2.1.2] play.core.router.Router#queryString causes an error against a parameter named "queryString" in routes

If I have a parameter named "queryString" in routes as below, I encounter an error.

* routes:

`GET     /                           controllers.Application.index()
GET     /hello                      controllers.Application.hello(queryString: String ?= null)
GET     /assets/*file               controllers.Assets.at(path="/public", file)`

* error:

`[info] Compiling 4 Scala sources and 2 Java sources to /home/elisa/repos/mine/play212Test/target/scala-2.10/classes...
[error] /home/elisa/repos/mine/play212Test/conf/routes:8: type mismatch;
[error]  found   : List[Option[String]]
[error]  required: Int
[error] GET     /hello                      controllers.Application.hello(queryString: String ?= null)
[error] one error found
[error] (compile:compile) Compilation failed
[error] application -`

It turned out that play.core.router.Router#queryString caused that error.

Can it be fixed?