diff --git a/fractor-xml/src/DomDocumentIterator.php b/fractor-xml/src/DomDocumentIterator.php
index dd5e48e8..9a91980a 100644
--- a/fractor-xml/src/DomDocumentIterator.php
+++ b/fractor-xml/src/DomDocumentIterator.php
@@ -20,7 +20,9 @@ public function traverseDocument(\DOMDocument $document): void
             $visitor->beforeTraversal($document);
         }
 
-        $this->traverseNode($document->firstChild);
+        if ($document->firstChild instanceof \DOMNode) {
+            $this->traverseNode($document->firstChild);
+        }
 
         foreach ($this->visitors as $visitor) {
             $visitor->afterTraversal($document);
@@ -33,6 +35,11 @@ private function traverseNode(\DOMNode $node): void
         foreach ($this->visitors as $visitor) {
             $result = $visitor->enterNode($node);
 
+            if ($node->parentNode === null) {
+                // TODO convert into a custom ShouldNotHappenException
+                throw new \RuntimeException('Node has no parent node');
+            }
+
             if ($result === DomDocumentIterator::REMOVE_NODE) {
                 $node->parentNode->removeChild($node);
                 $nodeRemoved = true;
diff --git a/fractor-xml/src/XmlFractor.php b/fractor-xml/src/XmlFractor.php
index 719bb89f..13f34730 100644
--- a/fractor-xml/src/XmlFractor.php
+++ b/fractor-xml/src/XmlFractor.php
@@ -2,9 +2,12 @@
 
 namespace a9f\FractorXml;
 
-interface XmlFractor
+interface XmlFractor extends DomNodeVisitor
 {
     public function canHandle(\DOMNode $node): bool;
 
+    /**
+     * @return \DOMNode|DomDocumentIterator::*|null
+     */
     public function refactor(\DOMNode $node): \DOMNode|int|null;
 }
diff --git a/fractor-xml/tests/DomDocumentIteratorTest.php b/fractor-xml/tests/DomDocumentIteratorTest.php
index 9b35aeb0..a552870d 100644
--- a/fractor-xml/tests/DomDocumentIteratorTest.php
+++ b/fractor-xml/tests/DomDocumentIteratorTest.php
@@ -168,7 +168,7 @@ public function enterNode(\DOMNode $node): \DOMNode|int
             'afterTraversal:#document',
         ], $nodeRemovingVisitor->calls);
 
-        self::assertXmlStringEqualsXmlString('<Root></Root>', $document->saveXML());
+        self::assertXmlStringEqualsXmlString('<Root></Root>', $document->saveXML() ?: '');
     }
 
     #[Test]
@@ -179,6 +179,10 @@ public function enterNode(\DOMNode $node): \DOMNode|int
             {
                 parent::enterNode($node);
                 if ($node->nodeName === 'Child') {
+                    if ($node->ownerDocument === null) {
+                        throw new \RuntimeException('Node does not have an ownerDocument, cannot create element');
+                    }
+
                     return $node->ownerDocument->createElement('NewChild');
                 }
                 return $node;
@@ -201,14 +205,17 @@ public function enterNode(\DOMNode $node): \DOMNode|int
             'afterTraversal:#document',
         ], $nodeRemovingVisitor->calls);
 
-        self::assertXmlStringEqualsXmlString('<Root><NewChild></NewChild></Root>', $document->saveXML());
+        self::assertXmlStringEqualsXmlString('<Root><NewChild></NewChild></Root>', $document->saveXML() ?: '');
     }
 
-    private function getCollectingDomNodeVisitor(): DomNodeVisitor
+    private function getCollectingDomNodeVisitor(): CollectingDomNodeVisitor
     {
         return new CollectingDomNodeVisitor();
     }
 
+    /**
+     * @param list<string> $recorder
+     */
     private function getCallRecordingDomNodeVisitor(string $visitorName, array &$recorder): DomNodeVisitor
     {
         return new class($visitorName, $recorder) implements DomNodeVisitor {
@@ -217,7 +224,7 @@ private function getCallRecordingDomNodeVisitor(string $visitorName, array &$rec
              */
             public function __construct(
                 private readonly string $visitorName,
-                private array &$calls
+                public array &$calls // only public to please PHPStan
             ) {
             }
 
diff --git a/fractor/src/DependencyInjection/ContainerBuilder.php b/fractor/src/DependencyInjection/ContainerBuilder.php
index 11f202f0..7b176c7d 100644
--- a/fractor/src/DependencyInjection/ContainerBuilder.php
+++ b/fractor/src/DependencyInjection/ContainerBuilder.php
@@ -30,7 +30,7 @@ public function createDependencyInjectionContainer(?string $fractorConfigFile):
         $config->addCompilerPass(new CommandsCompilerPass());
         $config->addCompilerPass(new FileProcessorCompilerPass());
 
-        if (is_file($fractorConfigFile)) {
+        if ($fractorConfigFile !== null && is_file($fractorConfigFile)) {
             $config->import($fractorConfigFile);
         }
 
@@ -42,7 +42,7 @@ public function createDependencyInjectionContainer(?string $fractorConfigFile):
         return $config;
     }
 
-    private function registerConfiguredRules(FractorConfig $config)
+    private function registerConfiguredRules(FractorConfig $config): void
     {
         foreach ($config->getRules() as $rule) {
             $config->registerForAutoconfiguration($rule)
@@ -50,7 +50,7 @@ private function registerConfiguredRules(FractorConfig $config)
         }
     }
 
-    private function registerConfiguredFileProcessors(FractorConfig $config)
+    private function registerConfiguredFileProcessors(FractorConfig $config): void
     {
         foreach ($config->getFileProcessors() as $processor) {
             $config->registerForAutoconfiguration($processor)
diff --git a/fractor/src/FileSystem/FileFinder.php b/fractor/src/FileSystem/FileFinder.php
index 7499b804..fa3225ee 100644
--- a/fractor/src/FileSystem/FileFinder.php
+++ b/fractor/src/FileSystem/FileFinder.php
@@ -7,6 +7,8 @@
 final class FileFinder
 {
     /**
+     * @param list<non-empty-string> $directories
+     * @param list<non-empty-string> $fileExtensions
      * @return list<\SplFileInfo>
      */
     public function findFiles(array $directories, array $fileExtensions): array
diff --git a/fractor/tests/Configuration/FractorConfigTest.php b/fractor/tests/Configuration/FractorConfigTest.php
index 468ef164..dc28a4f3 100644
--- a/fractor/tests/Configuration/FractorConfigTest.php
+++ b/fractor/tests/Configuration/FractorConfigTest.php
@@ -48,6 +48,7 @@ public function importFileThrowsExceptionIfNoClosureIsReturned(): void
     private function placeConfigFileInTemporaryFolderAndImport(FractorConfig $config, string $closure): void
     {
         $tempFile = tempnam(sys_get_temp_dir(), 'fractor-test');
+        self::assertIsString($tempFile);
         try {
             file_put_contents($tempFile, $closure);
 
diff --git a/typo3-fractor/composer.json b/typo3-fractor/composer.json
index 6126286f..479d8de9 100644
--- a/typo3-fractor/composer.json
+++ b/typo3-fractor/composer.json
@@ -14,7 +14,8 @@
         "php": "^8.2",
         "a9f/fractor": "@dev",
         "a9f/fractor-extension-installer": "@dev",
-        "a9f/fractor-xml": "@dev"
+        "a9f/fractor-xml": "@dev",
+        "thecodingmachine/safe": "^2.5"
     },
     "require-dev": {
         "ergebnis/composer-normalize": "^2.42",
diff --git a/typo3-fractor/src/AbstractFlexformFractor.php b/typo3-fractor/src/AbstractFlexformFractor.php
index c0e9012a..92520d01 100644
--- a/typo3-fractor/src/AbstractFlexformFractor.php
+++ b/typo3-fractor/src/AbstractFlexformFractor.php
@@ -8,6 +8,13 @@ abstract class AbstractFlexformFractor extends AbstractXmlFractor
 {
     public function canHandle(\DOMNode $node): bool
     {
-        return $node->ownerDocument->firstChild->nodeName === 'T3DataStructure';
+        $rootNode = $node->ownerDocument?->firstChild;
+
+        if ($rootNode === null) {
+            // TODO convert into a custom ShouldNotHappenException
+            throw new \RuntimeException('Node\'s document does not have a root node');
+        }
+
+        return $rootNode->nodeName === 'T3DataStructure';
     }
 }
diff --git a/typo3-fractor/src/Rules/FlexForm/AddRenderTypeToFlexFormFractor.php b/typo3-fractor/src/Rules/FlexForm/AddRenderTypeToFlexFormFractor.php
index cf2ffd4f..67db2305 100644
--- a/typo3-fractor/src/Rules/FlexForm/AddRenderTypeToFlexFormFractor.php
+++ b/typo3-fractor/src/Rules/FlexForm/AddRenderTypeToFlexFormFractor.php
@@ -37,7 +37,14 @@ public function refactor(\DOMNode $node): \DOMNode|int|null
         }
 
         if ($isSingleSelect) {
-            $renderType = $childNode->ownerDocument->createElement('renderType');
+            $ownerDocument = $node->ownerDocument;
+
+            if ($ownerDocument === null) {
+                // TODO convert into a custom ShouldNotHappenException
+                throw new \RuntimeException('Node does not have an ownerDocument');
+            }
+
+            $renderType = $ownerDocument->createElement('renderType');
             $renderType->nodeValue = 'selectSingle';
             $node->appendChild($renderType);
         }
diff --git a/typo3-fractor/tests/Rules/FlexForm/AddRenderTypeToFlexFormFractorTest.php b/typo3-fractor/tests/Rules/FlexForm/AddRenderTypeToFlexFormFractorTest.php
index 287ca224..2a50c2a1 100644
--- a/typo3-fractor/tests/Rules/FlexForm/AddRenderTypeToFlexFormFractorTest.php
+++ b/typo3-fractor/tests/Rules/FlexForm/AddRenderTypeToFlexFormFractorTest.php
@@ -7,12 +7,14 @@
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
+use function Safe\file_get_contents;
+
 class AddRenderTypeToFlexFormFractorTest extends TestCase
 {
     private const FIXTURE_SEPARATOR = '-----';
 
     /**
-     * @return array<string, string>
+     * @return array<string, array{string}>
      */
     public static function fixtureFilesProvider(): array
     {
@@ -41,7 +43,7 @@ public function test(string $filePath): void
         $iterator = new DomDocumentIterator([new AddRenderTypeToFlexFormFractor()]);
         $iterator->traverseDocument($document);
 
-        $result = $document->saveXML();
+        $result = $document->saveXML() ?: '';
 
         self::assertXmlStringEqualsXmlString($expectedResultXml, $result);
     }