problem description

Middleware $request- > header ($key) cannot get a value

the environmental background of the problems and what methods you have tried

lumen was good before, but I don"t know why recently
tried $request- > headers- > aaa
tried $request- > header ("aaa")
other values can be obtained, but header can"t get

related codes

/ / Please paste the code text below (do not replace the code with pictures)

public function handle($request, Closure $next)
{
    var_dump($request->headers->get("aaa"));
    var_dump($request->header("aaa"));
}

what result do you expect? What is the error message actually seen?

expectation: valuable
error: null