diff --git a/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php b/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
index 44a14352cd..ee476633a9 100644
--- a/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
+++ b/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright 2018 Morris Jobke <hey@morrisjobke.de>
  *
diff --git a/core/BackgroundJobs/CheckForUserCertificates.php b/core/BackgroundJobs/CheckForUserCertificates.php
index 8b106c8ce7..d8892e03b5 100644
--- a/core/BackgroundJobs/CheckForUserCertificates.php
+++ b/core/BackgroundJobs/CheckForUserCertificates.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright 2020 Morris Jobke <hey@morrisjobke.de>
  *
@@ -49,7 +52,7 @@ class CheckForUserCertificates extends QueuedJob {
 	/**
 	 * Checks all user directories for old user uploaded certificates
 	 */
-	public function run($arguments) {
+	public function run($arguments): void {
 		$uploadList = [];
 		$this->userManager->callForSeenUsers(function (IUser $user) use (&$uploadList) {
 			$userId = $user->getUID();
diff --git a/core/BackgroundJobs/CleanupLoginFlowV2.php b/core/BackgroundJobs/CleanupLoginFlowV2.php
index 9fee3550c8..69e7fe7195 100644
--- a/core/BackgroundJobs/CleanupLoginFlowV2.php
+++ b/core/BackgroundJobs/CleanupLoginFlowV2.php
@@ -42,7 +42,7 @@ class CleanupLoginFlowV2 extends TimedJob {
 		$this->setInterval(3600);
 	}
 
-	protected function run($argument) {
+	protected function run($argument): void {
 		$this->loginFlowV2Mapper->cleanup();
 	}
 }
diff --git a/core/register_command.php b/core/register_command.php
index 2a68cbcbe8..4d0e0ceb99 100644
--- a/core/register_command.php
+++ b/core/register_command.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/core/routes.php b/core/routes.php
index 5bc4075315..791fe8c74f 100644
--- a/core/routes.php
+++ b/core/routes.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/core/strings.php b/core/strings.php
index f59b5c8265..fa23fb1ac6 100644
--- a/core/strings.php
+++ b/core/strings.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *