Rails - Feb 2006 - Functional testing - why can you use session[:key] but not session[:key]= ?

Question''s all in the title..

def test_foo
  assert_equal ''foo'', session[:bar]  # Perfectly valid
end

def test_blowup
  session[:bar] = foo # TypeError: Symbol as array index
  ...
end

I always create a @session variable in the setup method.  Just curious
as to why you can get session values but not set them using session[]

Pat