diff --git a/lib/Component/VCard.php b/lib/Component/VCard.php
index bbb852cc2..0b8696fd4 100644
--- a/lib/Component/VCard.php
+++ b/lib/Component/VCard.php
@@ -479,7 +479,7 @@ public function xmlSerialize(Xml\Writer $writer): void
         $propertiesByGroup = [];
 
         foreach ($this->children() as $property) {
-            $group = $property->group;
+            $group = $property->group ?? '';
 
             $propertiesByGroup[$group] ??= [];
 
diff --git a/tests/phpunit.xml b/tests/phpunit.xml
index 6d940533a..8afdb308d 100644
--- a/tests/phpunit.xml
+++ b/tests/phpunit.xml
@@ -1,5 +1,14 @@
 <?xml version="1.0"?>
-<phpunit colors="true" bootstrap="bootstrap.php" beStrictAboutTestsThatDoNotTestAnything="true" beStrictAboutOutputDuringTests="true" failOnRisky="true" failOnWarning="true" enforceTimeLimit="true" cacheDirectory=".phpunit.cache">
+<phpunit colors="true"
+         bootstrap="bootstrap.php"
+         beStrictAboutTestsThatDoNotTestAnything="true"
+         beStrictAboutOutputDuringTests="true"
+         failOnRisky="true"
+         failOnWarning="true"
+         failOnDeprecation="true"
+         enforceTimeLimit="true"
+         cacheDirectory=".phpunit.cache"
+>
   <php>
     <ini name="error_reporting" value="E_ALL"/>
   </php>