Security exception when cache is passed as view parameter

Using 4.4.6, I'm getting a "CodeIgniter\Security\Exceptions\SecurityException #403" on a POST, when I submit a form. This only happens if I load the view(s) like this:

PHP Code:

$view_cache = ['cache' => nn]
return view( 'templates/header', $data, $view_cache )
         . view( 'login/signup', $data, $view_cache )
         . view( 'templates/footer', $data, $view_cache ); 


It doesn't matter if I set nn to zero or 60. If I change "cache" to "xcache" (which I'm guessing is simply ignored), everything works.

What am I doing wrong?

-joho