diff --git a/lib/ext/Syncroton/Server.php b/lib/ext/Syncroton/Server.php
--- a/lib/ext/Syncroton/Server.php
+++ b/lib/ext/Syncroton/Server.php
@@ -147,6 +147,13 @@
             if (!$response) {
                 $response = $command->getResponse();
             }
+
+            if ($response === null) {
+                // FIXME: Don't send any content-type, but will it not be added by the http server?
+                ini_set('default_mimetype', '');
+                // FIXME: Is this really needed? It is for PHP cli-server, but not really for a real http server
+                header('Content-Length: 0');
+            }
         } catch (Syncroton_Exception_ProvisioningNeeded $sepn) {
             if ($this->_logger instanceof Zend_Log) 
                 $this->_logger->info(__METHOD__ . '::' . __LINE__ . " provisioning needed");
diff --git a/tests/Sync/Sync/EmailTest.php b/tests/Sync/Sync/EmailTest.php
--- a/tests/Sync/Sync/EmailTest.php
+++ b/tests/Sync/Sync/EmailTest.php
@@ -112,29 +112,7 @@
         $this->assertSame('test sync', $xpath->query("{$root}/ns:ApplicationData/Email:Subject")->item(0)->nodeValue);
 
         // List the rest of the mail
-        $request = <<<EOF
-            <?xml version="1.0" encoding="utf-8"?>
-            <!DOCTYPE AirSync PUBLIC "-//AIRSYNC//DTD AirSync//EN" "http://www.microsoft.com/">
-            <Sync xmlns="uri:AirSync" xmlns:AirSyncBase="uri:AirSyncBase">
-                <Collections>
-                    <Collection>
-                        <SyncKey>{$syncKey}</SyncKey>
-                        <CollectionId>{$folderId}</CollectionId>
-                        <DeletesAsMoves>1</DeletesAsMoves>
-                        <GetChanges>1</GetChanges>
-                        <Options>
-                            <FilterType>0</FilterType>
-                            <Conflict>1</Conflict>
-                            <BodyPreference xmlns="uri:AirSyncBase">
-                                <Type>2</Type>
-                                <TruncationSize>51200</TruncationSize>
-                                <AllOrNone>0</AllOrNone>
-                            </BodyPreference>
-                        </Options>
-                    </Collection>
-                </Collections>
-            </Sync>
-            EOF;
+        $request = preg_replace('|<SyncKey>\d</SyncKey>|', "<SyncKey>{$syncKey}</SyncKey>", $request);
 
         $response = $this->request($request, 'Sync');
 
@@ -154,6 +132,24 @@
         $this->assertStringMatchesFormat("{$folderId}::%d", $xpath->query("{$root}/ns:ServerId")->item(0)->nodeValue);
         $this->assertSame('sync test with attachment', $xpath->query("{$root}/ns:ApplicationData/Email:Subject")->item(0)->nodeValue);
 
+        // Test the empty Sync response
+        $request = preg_replace('|<SyncKey>\d</SyncKey>|', "<SyncKey>{$syncKey}</SyncKey>", $request);
+
+        $response = $this->request($request, 'Sync');
+
+        print_r($response->getHeaders());
+
+        // According to https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-ascmd/b4b366a5-7dfb-45a9-a256-af8fa7c53400
+        // the empty response should look like this
+        //
+        // HTTP/1.1 200 OK
+        // Date: Fri, 10 Apr 2009 20:32:39 GMT
+        // Content-Length: 0
+
+        $this->assertSame(200, $response->getStatusCode());
+        $this->assertSame('', (string) $response->getBody());
+        $this->assertSame(['0'], $response->getHeader('Content-Length'));
+
         return $syncKey;
     }
 
diff --git a/tests/SyncTestCase.php b/tests/SyncTestCase.php
--- a/tests/SyncTestCase.php
+++ b/tests/SyncTestCase.php
@@ -360,16 +360,15 @@
         return $xpath;
     }
 
-
     /**
      * adapter for phpunit < 9
      */
-    protected function assertMatchesRegularExpression($arg1, $arg2)
+    public static function assertMatchesRegularExpression(string $arg1, string $arg2, string $message = ''): void
     {
         if (method_exists("PHPUnit\Framework\TestCase", "assertMatchesRegularExpression")) {
-            parent::assertMatchesRegularExpression($arg1, $arg2);
+            parent::assertMatchesRegularExpression($arg1, $arg2, $message);
         } else {
-            parent::assertRegExp($arg1, $arg2);
+            parent::assertRegExp($arg1, $arg2, $message);
         }
     }
 }