Setting a custom header in a functional test

According to the RoR testing guide I should be able to set a custom header in a functional test.

If I set it this way

@request.headers["Authorization"] = "Bearer ....."

everything works just.

If I try to dynamically compute header value and assign it to @request.headers["Authorization"]

jwt_token = AuthToken.encode('some string')
bearer = "Bearer #{jwt_token}"

@request.headers["Authorization"] = bearer

I get following error

NoMethodError: undefined method `request=' for nil:NilClass

Why does this happen? How can I avoid this problem?