Dusk - asserting route path is returns 'route not defined'

$browser
				->visit(route('posts.show', ['post' => $old->slug]))
				->screenshot(1)
				->assertRouteIs(route('posts.show', ['posts' => $old->slug]));

so when i see screenshot, I can see dusk has gone to the url - ie correct screenshot.. however assertRoute is.. returns

Route [http://laravel.test/posts/environment-neque-as-6914e1669609c] not defined.

  at vendor/laravel/framework/src/Illuminate/Routing/UrlGenerator.php:526
    522▕             ! is_null($url = call_user_func($this->missingNamedRouteResolver, $name, $parameters, $absolute))) {
    523▕             return $url;
    524▕         }
    525▕ 
  ➜ 526▕         throw new RouteNotFoundException("Route [{$name}] not defined.");
    527▕     }
    528▕ 
    529▕     /**
    530▕      * Get the URL for a given route instance.