diff --git a/src/Clover/Dto/ClassDto.php b/src/Clover/Dto/ClassDto.php
index 3df1fa5..a193733 100644
--- a/src/Clover/Dto/ClassDto.php
+++ b/src/Clover/Dto/ClassDto.php
@@ -14,6 +14,9 @@ class ClassDto
     ) {
     }
 
+    /**
+     * @return array<string, string>
+     */
     public function getProperties(): array
     {
         return $this->properties;
diff --git a/src/Clover/Dto/LineDto.php b/src/Clover/Dto/LineDto.php
index 8f8dea6..59c8a1c 100644
--- a/src/Clover/Dto/LineDto.php
+++ b/src/Clover/Dto/LineDto.php
@@ -22,6 +22,9 @@ class LineDto
         return $this->count;
     }
 
+    /**
+     * @return array<string, string>
+     */
     public function getProperties(): array
     {
         return $this->properties;
diff --git a/src/Clover/Parser.php b/src/Clover/Parser.php
index 3aefdb2..2f44fdc 100644
--- a/src/Clover/Parser.php
+++ b/src/Clover/Parser.php
@@ -57,6 +57,10 @@ class Parser implements ElementsDictionary
         return new LineDto($properties, (int) $count);
     }
 
+    /**
+     * @param \SimpleXMLElement $xml
+     * @return array<array-key, \SimpleXMLElement>
+     */
     public function getAttributes(\SimpleXMLElement $xml): array
     {
         return ((array) $xml->attributes())['@attributes'] ?? [];
diff --git a/src/Clover/Renderer.php b/src/Clover/Renderer.php
index 5bc8cec..cdad8b1 100644
--- a/src/Clover/Renderer.php
+++ b/src/Clover/Renderer.php
@@ -14,7 +14,7 @@ class Renderer
 {
     /**
      * @param Accumulator $accumulator
-     * @return array
+     * @return array{0: string, 1: Metrics}
      * @throws \DOMException
      */
     public function renderAccumulator(Accumulator $accumulator): array
@@ -71,7 +71,7 @@ class Renderer
 
         $xmlProject->appendChild($this->renderMetricsProject($xmlDocument, $projectMetrics));
 
-        return [$xmlDocument->saveXML(), $projectMetrics];
+        return [(string) $xmlDocument->saveXML(), $projectMetrics];
     }
 
     /**
diff --git a/src/Command/CloverMergeCommand.php b/src/Command/CloverMergeCommand.php
index 943835c..146daab 100644
--- a/src/Command/CloverMergeCommand.php
+++ b/src/Command/CloverMergeCommand.php
@@ -56,6 +56,11 @@ final class CloverMergeCommand extends CliCommand
     }
 
     /**
+     * @param array<array-key, string> $files
+     * @param string $output
+     * @param string|null $workdir
+     * @param bool $stats
+     * @return int
      * @throws ExecuteException
      */
     public function execute(array $files, string $output, ?string $workdir, bool $stats): int
diff --git a/tests/Unit/Clover/Handler/HandleSingleDocumentTest.php b/tests/Unit/Clover/Handler/HandleSingleDocumentTest.php
index d9819e0..0874b7f 100644
--- a/tests/Unit/Clover/Handler/HandleSingleDocumentTest.php
+++ b/tests/Unit/Clover/Handler/HandleSingleDocumentTest.php
@@ -10,11 +10,11 @@ use Dannecron\CoverageMerger\Clover\Parser;
 \test('examples without files', function (string $exampleFilename): void {
     $handler = new Handler(new Parser());
 
-    $cloverContents = \file_get_contents(\getExamplePath($exampleFilename));
+    $cloverContents = (string) \file_get_contents(\getExamplePath($exampleFilename));
+    /** @var \SimpleXMLElement $xml */
+    $xml = \simplexml_load_string($cloverContents);
 
-    $accumulator = $handler->handleSingleDocument(
-        \simplexml_load_string($cloverContents),
-    );
+    $accumulator = $handler->handleSingleDocument($xml);
 
     $files = $accumulator->getFiles();
     \expect($files)->toHaveCount(0);
@@ -34,11 +34,11 @@ use Dannecron\CoverageMerger\Clover\Parser;
 ): void {
     $handler = new Handler(new Parser());
 
-    $cloverContents = \file_get_contents(\getExamplePath($exampleFilename));
+    $cloverContents = (string) \file_get_contents(\getExamplePath($exampleFilename));
+    /** @var \SimpleXMLElement $xml */
+    $xml = \simplexml_load_string($cloverContents);
 
-    $accumulator = $handler->handleSingleDocument(
-        \simplexml_load_string($cloverContents),
-    );
+    $accumulator = $handler->handleSingleDocument($xml);
 
     $files = $accumulator->getFiles();
     \expect($files)->toHaveCount(1)->toHaveKey($expectedFilename);
@@ -62,11 +62,11 @@ use Dannecron\CoverageMerger\Clover\Parser;
 ): void {
     $handler = new Handler(new Parser());
 
-    $cloverContents = \file_get_contents(\getExamplePath($exampleFilename));
+    $cloverContents = (string) \file_get_contents(\getExamplePath($exampleFilename));
+    /** @var \SimpleXMLElement $xml */
+    $xml = \simplexml_load_string($cloverContents);
 
-    $accumulator = $handler->handleSingleDocument(
-        \simplexml_load_string($cloverContents),
-    );
+    $accumulator = $handler->handleSingleDocument($xml);
 
     $files = $accumulator->getFiles();
     \expect($files)->toHaveCount(2)
@@ -81,12 +81,12 @@ use Dannecron\CoverageMerger\Clover\Parser;
 \test('examples with invalid structure', function (string $exampleFilename): void {
     $handler = new Handler(new Parser());
 
-    $cloverContents = \file_get_contents(\getExamplePath($exampleFilename));
+    $cloverContents = (string) \file_get_contents(\getExamplePath($exampleFilename));
+    /** @var \SimpleXMLElement $xml */
+    $xml = \simplexml_load_string($cloverContents);
 
     $this->expectException(HandleException::class);
-    $handler->handleSingleDocument(
-        \simplexml_load_string($cloverContents),
-    );
+    $handler->handleSingleDocument($xml);
 })
     ->with([
         'file-with-bad-line.xml',
diff --git a/tests/Unit/Clover/Handler/HandleTest.php b/tests/Unit/Clover/Handler/HandleTest.php
index 2a2101d..61fc0f1 100644
--- a/tests/Unit/Clover/Handler/HandleTest.php
+++ b/tests/Unit/Clover/Handler/HandleTest.php
@@ -9,18 +9,27 @@ use Dannecron\CoverageMerger\Clover\Handler;
 use Dannecron\CoverageMerger\Clover\Parser;
 
 \test('merge multiple valid files', function (): void {
-    $fileWithPackage = \file_get_contents(\getExamplePath('file-with-package.xml'));
-    $fileWithoutPackage = \file_get_contents(\getExamplePath('file-without-package.xml'));
-    $fileWithDifferences = \file_get_contents(\getExamplePath('file-with-differences.xml'));
-    $metricsAndClasses = \file_get_contents(\getExamplePath('metrics-and-classes.xml'));
+    $fileWithPackage = (string) \file_get_contents(\getExamplePath('file-with-package.xml'));
+    $fileWithoutPackage = (string) \file_get_contents(\getExamplePath('file-without-package.xml'));
+    $fileWithDifferences = (string) \file_get_contents(\getExamplePath('file-with-differences.xml'));
+    $metricsAndClasses = (string) \file_get_contents(\getExamplePath('metrics-and-classes.xml'));
 
     $handler = new Handler(new Parser());
 
+    /** @var \SimpleXMLElement $xmlFileWithPackage */
+    $xmlFileWithPackage = \simplexml_load_string($fileWithPackage);
+    /** @var \SimpleXMLElement $xmlFileWithoutPackage */
+    $xmlFileWithoutPackage = \simplexml_load_string($fileWithoutPackage);
+    /** @var \SimpleXMLElement $xmlFileWithDifferences */
+    $xmlFileWithDifferences = \simplexml_load_string($fileWithDifferences);
+    /** @var \SimpleXMLElement $xmlMetricsAndClasses */
+    $xmlMetricsAndClasses = \simplexml_load_string($metricsAndClasses);
+
     $accumulator = $handler->handle(
-        \simplexml_load_string($fileWithPackage),
-        \simplexml_load_string($fileWithoutPackage),
-        \simplexml_load_string($fileWithDifferences),
-        \simplexml_load_string($metricsAndClasses),
+        $xmlFileWithPackage,
+        $xmlFileWithoutPackage,
+        $xmlFileWithDifferences,
+        $xmlMetricsAndClasses,
     );
 
     $files = $accumulator->getFiles();