Cakephp error handling goes haywire after Controller::viewVars and filtered through htmlentities

Using Cakephp 2.5.4 on a fresh cake code base.

I have my AppController Setup as follows.

class AppController extends Controller {
    public function beforeRender() {
        // Filter all output
        array_walk_recursive($this->viewVars, [$this, 'escapeOutput']);
        // throw new NotFoundException('invalid url');
    }

    public function escapeOutput(&$value, $key) {
        $value = htmlentities($value);
    }
}

This works as expected. However if I now throw an error (uncomment the line after array_walk) cake throws an extra error as follows.

Fatal error: Call to a member function getMessage() on a non-object in **/lib/Cake/View/Errors/fatal_error.ctp on line 21

Could someone explain what is going on?

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