diff --git a/lib/Controller/SettingsController.php b/lib/Controller/SettingsController.php
index acf3520..7b26a34 100644
--- a/lib/Controller/SettingsController.php
+++ b/lib/Controller/SettingsController.php
@@ -51,7 +51,7 @@ class SettingsController extends Controller {
      *
 	 * @return array|\OCP\AppFramework\Http\JSONResponse
 	 */
-    public function setPreference($EpubEnable, $PdfEnable, $CbxEnable) {
+    public function setPreference(string $EpubEnable, string $PdfEnable, string $CbxEnable) {
 
 		$l = \OC::$server->getL10N('epubreader');