(SETF (UIOP:GETENV VAR) NIL) should unset environment variable VAR

When an environment variable is not set, then UIOP:GETENV returns NIL when trying to retrieve the value of that variable. However NIL is not a supported value for (SETF UIOP:GETENV) so that the following code does not work as expected:

(flet ((some-action (value)
	 (declare (ignore value)))
       (ensure-environment-is-clean ()
	 (unless (eq nil (uiop:getenv "VAR"))
	   (error "The variable VAR should not be bound."))))
  (let ((saved-value
	  (uiop:getenv "VAR")))
    (ensure-environment-is-clean)
    (setf (uiop:getenv "VAR") "alternative value")
    (unwind-protect (some-action (uiop:getenv "VAR"))
      (setf (uiop:getenv "VAR") saved-value))
    (ensure-environment-is-clean)))

The expected behaviour is that the snippet of code above completes successfully but on SBCL it returns an error. I am not sure what is the behaviour of other implementations.