diff --git a/phpstan.neon b/phpstan.neon
index af2c49d..49cb743 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,4 +1,5 @@
 parameters:
   level: 8
   paths:
-    - src
\ No newline at end of file
+    - src
+    - tests
\ No newline at end of file
diff --git a/tests/ApiTest.php b/tests/ApiTest.php
index 5d4cb91..3c17e1e 100644
--- a/tests/ApiTest.php
+++ b/tests/ApiTest.php
@@ -2,6 +2,7 @@
 
 namespace Sysix\LexOffice\Tests;
 
+use Sysix\LexOffice\Api;
 use Sysix\LexOffice\Clients\Contact;
 use Sysix\LexOffice\Clients\Country;
 use Sysix\LexOffice\Clients\CreditNote;
@@ -19,9 +20,14 @@
 use Sysix\LexOffice\Clients\VoucherList;
 use Sysix\LexOffice\Clients\RecurringTemplate;
 use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\MockObject\MockObject;
 
 class ApiTest extends TestClient
 {
+    /**
+     * @param Response $response
+     * @return Api&MockObject
+     */
     public function createApiMockObject(Response $response)
     {
         $stub = parent::createApiMockObject($response);
@@ -30,7 +36,7 @@ public function createApiMockObject(Response $response)
         return $stub;
     }
 
-    public function testClients()
+    public function testClients(): void
     {
         $stub = $this->createApiMockObject(new Response());
 
@@ -52,7 +58,7 @@ public function testClients()
         $this->assertInstanceOf(PostingCategory::class, $stub->postingCategory());
     }
 
-    public function testApiUrl()
+    public function testApiUrl(): void
     {
         $stub = $this->createApiMockObject(
             new Response(200, [], 'post-content')
@@ -66,7 +72,7 @@ public function testApiUrl()
         $this->assertStringStartsWith('test.de', $stub->request->getUri()->getHost());
     }
 
-    public function testGetResponse()
+    public function testGetResponse(): void
     {
         $responseMock = new Response(200, [], 'response-body');
         $stub = $this->createApiMockObject($responseMock);
@@ -80,7 +86,7 @@ public function testGetResponse()
         $this->assertEquals($responseMock->getReasonPhrase(), $response->getReasonPhrase());
     }
 
-    public function testRequestHeaders()
+    public function testRequestHeaders(): void
     {
         $stub = $this->createApiMockObject(
             new Response(200, [], 'post-content')
diff --git a/tests/Clients/ContactTest.php b/tests/Clients/ContactTest.php
index b7efe57..eb4fd6c 100644
--- a/tests/Clients/ContactTest.php
+++ b/tests/Clients/ContactTest.php
@@ -8,7 +8,7 @@
 
 class ContactTest extends TestClient
 {
-    public function testGenerateUrl()
+    public function testGenerateUrl(): void
     {
         $stub = $this->createClientMockObject(
             Contact::class,
@@ -21,7 +21,7 @@ public function testGenerateUrl()
         );
     }
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub = $this->createClientMockObject(
             Contact::class,
@@ -35,7 +35,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGet()
+    public function testGet(): void
     {
         $stub = $this->createClientMockObject(
             Contact::class,
@@ -47,7 +47,7 @@ public function testGet()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $stub = $this->createClientMockObject(
             Contact::class,
diff --git a/tests/Clients/CountryTest.php b/tests/Clients/CountryTest.php
index 33bd2d8..46389bb 100644
--- a/tests/Clients/CountryTest.php
+++ b/tests/Clients/CountryTest.php
@@ -8,7 +8,7 @@
 
 class CountryTest extends TestClient
 {
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             Country::class,
diff --git a/tests/Clients/CreditNoteTest.php b/tests/Clients/CreditNoteTest.php
index 56ed25e..562e8da 100644
--- a/tests/Clients/CreditNoteTest.php
+++ b/tests/Clients/CreditNoteTest.php
@@ -8,7 +8,7 @@
 
 class CreditNoteTest extends TestClient
 {
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub = $this->createClientMockObject(
             CreditNote::class,
@@ -22,7 +22,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             CreditNote::class,
@@ -34,7 +34,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDocument()
+    public function testDocument(): void
     {
         $stub  = $this->createClientMockObject(
             CreditNote::class,
diff --git a/tests/Clients/DownPaymentInvoiceTest.php b/tests/Clients/DownPaymentInvoiceTest.php
index fb358fb..566a7af 100644
--- a/tests/Clients/DownPaymentInvoiceTest.php
+++ b/tests/Clients/DownPaymentInvoiceTest.php
@@ -8,7 +8,7 @@
 
 class DownPaymentInvoiceTest extends TestClient
 {
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             DownPaymentInvoice::class,
@@ -20,7 +20,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDocument()
+    public function testDocument(): void
     {
         $stub  = $this->createClientMockObject(
             DownPaymentInvoice::class,
diff --git a/tests/Clients/EventTest.php b/tests/Clients/EventTest.php
index 4a21bf5..eeee8d7 100644
--- a/tests/Clients/EventTest.php
+++ b/tests/Clients/EventTest.php
@@ -9,7 +9,7 @@
 class EventTest extends TestClient
 {
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub = $this->createClientMockObject(
             Event::class,
@@ -23,7 +23,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             Event::class,
@@ -35,7 +35,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDelete()
+    public function testDelete(): void
     {
         $stub = $this->createClientMockObject(
             Event::class,
diff --git a/tests/Clients/FileTest.php b/tests/Clients/FileTest.php
index c1f2bfa..9ad18a7 100644
--- a/tests/Clients/FileTest.php
+++ b/tests/Clients/FileTest.php
@@ -10,7 +10,7 @@
 
 class FileTest extends TestClient
 {
-    public function testUploadNotSupportedExtension()
+    public function testUploadNotSupportedExtension(): void
     {
         $this->expectException(LexOfficeApiException::class);
 
@@ -22,7 +22,7 @@ public function testUploadNotSupportedExtension()
         $stub->upload('not_allowed.gif', 'voucher');
     }
 
-    public function testUploadNotFound()
+    public function testUploadNotFound(): void
     {
         $this->expectException(LexOfficeApiException::class);
 
@@ -34,7 +34,7 @@ public function testUploadNotFound()
         $stub->upload('not_existing.jpg', 'voucher');
     }
 
-    public function testUploadToBig()
+    public function testUploadToBig(): void
     {
         $this->expectException(LexOfficeApiException::class);
 
@@ -45,7 +45,12 @@ public function testUploadToBig()
 
         $this->createCacheDir();
         $file = $this->getCacheDir() . '/somefile.jpg';
-        $fp = fopen($file, 'w+'); //
+        $fp = fopen($file, 'w+');
+
+        if ($fp === false) {
+            $this->fail('could not open file ' . $file);
+        }
+
         fseek($fp, File::MAX_FILE_SIZE + 1,SEEK_CUR);
         fwrite($fp,'a');
         fclose($fp);
@@ -55,7 +60,7 @@ public function testUploadToBig()
         unlink($file);
     }
 
-    public function testUpload()
+    public function testUpload(): void
     {
         $stub  = $this->createClientMockObject(
             File::class,
@@ -64,7 +69,12 @@ public function testUpload()
 
         $this->createCacheDir();
         $file = $this->getCacheDir() .  '/somefile2.jpg';
-        $fp = fopen($file, 'w+'); //
+        $fp = fopen($file, 'w+');
+
+        if ($fp === false) {
+            $this->fail('could not open file ' . $file);
+        }
+
         fseek($fp, 5,SEEK_CUR);
         fwrite($fp,'a');
         fclose($fp);
diff --git a/tests/Clients/InvoiceTest.php b/tests/Clients/InvoiceTest.php
index bba0ab3..794f69c 100644
--- a/tests/Clients/InvoiceTest.php
+++ b/tests/Clients/InvoiceTest.php
@@ -9,7 +9,7 @@
 class InvoiceTest extends TestClient
 {
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub = $this->createClientMockObject(
             Invoice::class,
@@ -23,7 +23,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGet()
+    public function testGet(): void
     {
         $stub = $this->createClientMockObject(
             Invoice::class,
@@ -35,7 +35,7 @@ public function testGet()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             Invoice::class,
@@ -47,7 +47,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDocument()
+    public function testDocument(): void
     {
         $stub  = $this->createClientMockObject(
             Invoice::class,
diff --git a/tests/Clients/OrderConfirmationTest.php b/tests/Clients/OrderConfirmationTest.php
index 6e44251..f5a9f44 100644
--- a/tests/Clients/OrderConfirmationTest.php
+++ b/tests/Clients/OrderConfirmationTest.php
@@ -9,7 +9,7 @@
 class OrderConfirmationTest extends TestClient
 {
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub  = $this->createClientMockObject(
             OrderConfirmation::class,
@@ -23,7 +23,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGet()
+    public function testGet(): void
     {
         $stub  = $this->createClientMockObject(
             OrderConfirmation::class,
@@ -35,7 +35,7 @@ public function testGet()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub  = $this->createClientMockObject(
             OrderConfirmation::class,
@@ -47,7 +47,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDocument()
+    public function testDocument(): void
     {
         $stub  = $this->createClientMockObject(
             OrderConfirmation::class,
diff --git a/tests/Clients/PaymentConditionTest.php b/tests/Clients/PaymentConditionTest.php
index 276bc82..0a3959c 100644
--- a/tests/Clients/PaymentConditionTest.php
+++ b/tests/Clients/PaymentConditionTest.php
@@ -8,7 +8,7 @@
 
 class PaymentConditionTest extends TestClient
 {
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             PaymentCondition::class,
diff --git a/tests/Clients/PaymentTest.php b/tests/Clients/PaymentTest.php
index a5ecb62..a2b179c 100644
--- a/tests/Clients/PaymentTest.php
+++ b/tests/Clients/PaymentTest.php
@@ -8,7 +8,7 @@
 
 class PaymentTest extends TestClient
 {
-    public function testGet()
+    public function testGet(): void
     {
         $stub = $this->createClientMockObject(
             Payment::class,
diff --git a/tests/Clients/ProfileTest.php b/tests/Clients/ProfileTest.php
index 4dcb70d..1ddc371 100644
--- a/tests/Clients/ProfileTest.php
+++ b/tests/Clients/ProfileTest.php
@@ -8,7 +8,7 @@
 
 class ProfileTest extends TestClient
 {
-    public function testGet()
+    public function testGet(): void
     {
         $stub = $this->createClientMockObject(
             Profile::class,
diff --git a/tests/Clients/QuotationTest.php b/tests/Clients/QuotationTest.php
index eea48ec..23d4a9d 100644
--- a/tests/Clients/QuotationTest.php
+++ b/tests/Clients/QuotationTest.php
@@ -9,7 +9,7 @@
 class QuotationTest extends TestClient
 {
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub = $this->createClientMockObject(
             Quotation::class,
@@ -23,7 +23,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGet()
+    public function testGet(): void
     {
         $stub = $this->createClientMockObject(
             Quotation::class,
@@ -35,7 +35,7 @@ public function testGet()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createClientMockObject(
             Quotation::class,
@@ -47,7 +47,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testDocument()
+    public function testDocument(): void
     {
         $stub = $this->createClientMockObject(
             Quotation::class,
diff --git a/tests/Clients/VoucherListTest.php b/tests/Clients/VoucherListTest.php
index b5d8e4a..7ba083b 100644
--- a/tests/Clients/VoucherListTest.php
+++ b/tests/Clients/VoucherListTest.php
@@ -9,7 +9,7 @@
 class VoucherListTest extends TestClient
 {
 
-    public function testGenerateUrl()
+    public function testGenerateUrl(): void
     {
         $stub = $this->createClientMockObject(
             VoucherList::class,
diff --git a/tests/Clients/VoucherTest.php b/tests/Clients/VoucherTest.php
index b250bf0..5ed5a6c 100644
--- a/tests/Clients/VoucherTest.php
+++ b/tests/Clients/VoucherTest.php
@@ -9,7 +9,7 @@
 class VoucherTest extends TestClient
 {
 
-    public function testCreate()
+    public function testCreate(): void
     {
         $stub  = $this->createClientMockObject(
             Voucher::class,
@@ -23,7 +23,7 @@ public function testCreate()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGet()
+    public function testGet(): void
     {
         $stub  = $this->createClientMockObject(
             Voucher::class,
@@ -35,7 +35,7 @@ public function testGet()
         $this->assertEquals('body', $response->getBody()->__toString());
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub  = $this->createClientMockObject(
             Voucher::class,
@@ -47,7 +47,7 @@ public function testGetAll()
         $this->assertEquals('{"content": [], "totalPages": 1}', $response->getBody()->__toString());
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $stub  = $this->createClientMockObject(
             Voucher::class,
diff --git a/tests/PaginationClientTest.php b/tests/PaginationClientTest.php
index 494434a..96c652e 100644
--- a/tests/PaginationClientTest.php
+++ b/tests/PaginationClientTest.php
@@ -7,7 +7,7 @@
 class PaginationClientTest extends TestClient
 {
 
-    public function testGenerateUrl()
+    public function testGenerateUrl(): void
     {
         $stub = $this->createPaginationClientMockObject(
             [new Response()]
@@ -19,7 +19,7 @@ public function testGenerateUrl()
         );
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $stub = $this->createPaginationClientMockObject(
             [new Response(200, [], '{"content": [], "totalPages": 1}')]
@@ -44,7 +44,7 @@ public function testGetAll()
         );
     }
 
-    public function testGetPage()
+    public function testGetPage(): void
     {
         $stub = $this->createPaginationClientMockObject(
             [new Response(200, [], '{"content": [], "totalPages": 1}')]
diff --git a/tests/TestClient.php b/tests/TestClient.php
index 17902bf..66e08a2 100644
--- a/tests/TestClient.php
+++ b/tests/TestClient.php
@@ -4,6 +4,7 @@
 
 use Sysix\LexOffice\Api;
 use Sysix\LexOffice\PaginationClient;
+use Sysix\LexOffice\ClientInterface;
 use GuzzleHttp\Client;
 use GuzzleHttp\Handler\MockHandler;
 use GuzzleHttp\HandlerStack;
@@ -17,7 +18,7 @@ class TestClient extends TestCase
 {
     /**
      * @param Response $response
-     * @return Api|MockObject
+     * @return Api&MockObject
      */
     public function createApiMockObject(Response $response)
     {
@@ -26,7 +27,7 @@ public function createApiMockObject(Response $response)
 
     /**
      * @param Response[] $responses
-     * @return MockObject|Api
+     * @return Api&MockObject
      */
     public function createApiMultiMockObject(array $responses)
     {
@@ -34,7 +35,7 @@ public function createApiMultiMockObject(array $responses)
 
         return $this
             ->getMockBuilder(Api::class)
-            ->addMethods([])
+            ->onlyMethods([])
             ->setConstructorArgs([
                 '',
                 new Client([
@@ -45,10 +46,10 @@ public function createApiMultiMockObject(array $responses)
     }
 
     /**
-     * @template T
+     * @template T of ClientInterface
      * @param class-string<T> $className
      * @param Response $response
-     * @return MockObject|T
+     * @return T&MockObject
      */
     public function createClientMockObject(string $className, Response $response)
     {
@@ -56,10 +57,10 @@ public function createClientMockObject(string $className, Response $response)
     }
 
     /**
-     * @template T
+     * @template T of ClientInterface
      * @param class-string<T> $className
      * @param Response[] $responses
-     * @return MockObject|T
+     * @return T&MockObject
      */
     public function createClientMultiMockObject(string $className, array $responses)
     {
@@ -67,14 +68,14 @@ public function createClientMultiMockObject(string $className, array $responses)
 
         return $this
             ->getMockBuilder($className)
-            ->addMethods([])
+            ->onlyMethods([])
             ->setConstructorArgs([$api])
             ->getMock();
     }
 
     /**
      * @param Response[] $responses
-     * @return MockObject|PaginationClient
+     * @return PaginationClient&MockObject
      * @throws ReflectionException
      */
     public function createPaginationClientMockObject(array $responses)
@@ -83,9 +84,9 @@ public function createPaginationClientMockObject(array $responses)
 
         $stub = $this
             ->getMockBuilder(PaginationClient::class)
-            ->addMethods([])
+            ->onlyMethods([])
             ->setConstructorArgs([$api])
-            ->getMockForAbstractClass();
+            ->getMock();
 
         $this->setProtectedProperty($stub, 'resource', 'resource');