diff --git a/rector.php b/rector.php
index aec92a3cd..5c76bdc78 100644
--- a/rector.php
+++ b/rector.php
@@ -8,6 +8,7 @@ declare(strict_types=1);
  */
 
 use Rector\Config\RectorConfig;
+use Rector\TypeDeclaration\Rector\Class_\AddTestsVoidReturnTypeWhereNoReturnRector;
 
 return RectorConfig::configure()
 	->withPaths([
@@ -23,4 +24,7 @@ return RectorConfig::configure()
 	)
 	->withPhpSets(
 		php73: true,
-	);
+	)
+	->withRules([
+		AddTestsVoidReturnTypeWhereNoReturnRector::class,
+	]);
diff --git a/tests/Unit/Service/Sync/SyncServiceTest.php b/tests/Unit/Service/Sync/SyncServiceTest.php
index d811ea457..f3a73375b 100644
--- a/tests/Unit/Service/Sync/SyncServiceTest.php
+++ b/tests/Unit/Service/Sync/SyncServiceTest.php
@@ -81,7 +81,7 @@ class SyncServiceTest extends TestCase {
 		);
 	}
 
-	public function testPartialSyncOnUncachedMailbox() {
+	public function testPartialSyncOnUncachedMailbox(): void {
 		$account = $this->createMock(Account::class);
 		$mailbox = $this->createMock(Mailbox::class);
 		$mailbox->expects($this->once())
@@ -100,7 +100,7 @@ class SyncServiceTest extends TestCase {
 		);
 	}
 
-	public function testSyncMailboxReturnsFolderStats() {
+	public function testSyncMailboxReturnsFolderStats(): void {
 		$account = $this->createMock(Account::class);
 		$account->method('getUserId')->willReturn('user');
 		$mailbox = new Mailbox();