diff --git a/src/Api.php b/src/Api.php
index 3f467c1..ad6b005 100644
--- a/src/Api.php
+++ b/src/Api.php
@@ -6,6 +6,7 @@
 
 
 use Baraja\AssetsLoader\Minifier\Minifier;
+use Nette\Utils\FileSystem;
 
 final class Api
 {
@@ -113,7 +114,7 @@ public function run(string $path): void
 			echo '/* Path "' . htmlspecialchars($parser[1]) . '" was automatically generated ' . date('Y-m-d H:i:s', $topModTime) . ' */' . "\n\n"; // 4.
 			foreach ($filePaths as $file => $filePath) {
 				echo '/* ' . $file . ' */' . "\n";
-				echo $this->minifier->minify(file_get_contents($filePath), $format);
+				echo $this->minifier->minify(FileSystem::read($filePath), $format);
 				echo "\n\n";
 			}
 			die;
diff --git a/src/Helpers.php b/src/Helpers.php
index f53d34d..ad57c00 100644
--- a/src/Helpers.php
+++ b/src/Helpers.php
@@ -48,7 +48,7 @@ public static function formatRoute(string $module, string $presenter = 'Homepage
 	}
 
 
-	public static function formatRouteToPath(string $route): ?string
+	public static function formatRouteToPath(string $route): string
 	{
 		if ($route === 'Error4xx:default') {
 			return 'error4xx-default';
@@ -57,7 +57,7 @@ public static function formatRouteToPath(string $route): ?string
 			return self::firstLower($parser['module']) . '-' . self::firstLower($parser['presenter']) . '-' . self::firstLower($parser['action']);
 		}
 
-		return null;
+		throw new \InvalidArgumentException('Can not parse route format, because haystack "' . $route . '" given.');
 	}
 
 
diff --git a/src/Minifier/DefaultCssMinifier.php b/src/Minifier/DefaultCssMinifier.php
index d0947e6..8b9ef91 100644
--- a/src/Minifier/DefaultCssMinifier.php
+++ b/src/Minifier/DefaultCssMinifier.php
@@ -9,7 +9,7 @@ final class DefaultCssMinifier implements AssetMinifier
 {
 	public function minify(string $haystack): string
 	{
-		return preg_replace_callback(
+		return (string) preg_replace_callback(
 			'#[ \t\r\n]+|<(/)?(textarea|pre|script)(?=\W)#si',
 			static function ($m): string {
 				if (empty($m[2])) {
diff --git a/src/Minifier/DefaultJsMinifier.php b/src/Minifier/DefaultJsMinifier.php
index 864f43e..493a1de 100644
--- a/src/Minifier/DefaultJsMinifier.php
+++ b/src/Minifier/DefaultJsMinifier.php
@@ -11,6 +11,6 @@ final class DefaultJsMinifier implements AssetMinifier
 {
 	public function minify(string $haystack): string
 	{
-		return JShrinkMinifier::minify($haystack);
+		return (string) JShrinkMinifier::minify($haystack);
 	}
 }