diff --git a/src/TestUtils/ArrayizableElementTestTrait.php b/src/TestUtils/ArrayizableElementTestTrait.php
index c1e7113..8a3eecf 100644
--- a/src/TestUtils/ArrayizableElementTestTrait.php
+++ b/src/TestUtils/ArrayizableElementTestTrait.php
@@ -15,11 +15,11 @@
  */
 trait ArrayizableElementTestTrait
 {
-    /** @var class-string|null */
-    protected static ?string $testedClass;
+    /** @var class-string */
+    protected static string $testedClass;
 
-    /** @var array|null */
-    protected static ?array $arrayRepresentation;
+    /** @var array */
+    protected static array $arrayRepresentation;
 
 
     /**
diff --git a/src/TestUtils/SchemaValidationTestTrait.php b/src/TestUtils/SchemaValidationTestTrait.php
index 4db9ea8..e7474dc 100644
--- a/src/TestUtils/SchemaValidationTestTrait.php
+++ b/src/TestUtils/SchemaValidationTestTrait.php
@@ -25,14 +25,14 @@
  */
 trait SchemaValidationTestTrait
 {
-    /** @var class-string|null */
-    protected static ?string $testedClass;
+    /** @var class-string */
+    protected static string $testedClass;
 
-    /** @var string|null */
-    protected static ?string $schemaFile;
+    /** @var string */
+    protected static string $schemaFile;
 
-    /** @var \DOMDocument|null */
-    protected static ?DOMDocument $xmlRepresentation;
+    /** @var \DOMDocument */
+    protected static DOMDocument $xmlRepresentation;
 
 
     /**
diff --git a/src/TestUtils/SerializableElementTestTrait.php b/src/TestUtils/SerializableElementTestTrait.php
index 916001e..4fb7d9c 100644
--- a/src/TestUtils/SerializableElementTestTrait.php
+++ b/src/TestUtils/SerializableElementTestTrait.php
@@ -15,11 +15,11 @@
  */
 trait SerializableElementTestTrait
 {
-    /** @var class-string|null */
-    protected static ?string $testedClass;
+    /** @var class-string */
+    protected static string $testedClass;
 
-    /** @var \DOMDocument|null */
-    protected static ?DOMDocument $xmlRepresentation;
+    /** @var \DOMDocument */
+    protected static DOMDocument $xmlRepresentation;
 
 
     /**