diff --git a/phpstan.neon b/phpstan.neon
index fddfd9c..772c5cc 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,5 +1,5 @@
 parameters:
-    level: 3
+    level: 4
     paths:
         - src
         - tests
diff --git a/src/Spout/Common/Entity/Style/Style.php b/src/Spout/Common/Entity/Style/Style.php
index b5f38f9..0af1450 100644
--- a/src/Spout/Common/Entity/Style/Style.php
+++ b/src/Spout/Common/Entity/Style/Style.php
@@ -66,7 +66,7 @@ class Style
     /** @var bool Whether the wrap text property was set */
     private $hasSetWrapText = false;
 
-    /** @var Border */
+    /** @var Border|null */
     private $border;
 
     /** @var bool Whether border properties should be applied */
@@ -78,7 +78,7 @@ class Style
     /** @var bool */
     private $hasSetBackgroundColor = false;
 
-    /** @var string Format */
+    /** @var string|null Format */
     private $format;
 
     /** @var bool */
@@ -110,7 +110,7 @@ class Style
     }
 
     /**
-     * @return Border
+     * @return Border|null
      */
     public function getBorder()
     {
@@ -467,7 +467,7 @@ class Style
     }
 
     /**
-     * @return string
+     * @return string|null
      */
     public function getFormat()
     {
diff --git a/src/Spout/Common/Helper/GlobalFunctionsHelper.php b/src/Spout/Common/Helper/GlobalFunctionsHelper.php
index c5c92bc..5068227 100644
--- a/src/Spout/Common/Helper/GlobalFunctionsHelper.php
+++ b/src/Spout/Common/Helper/GlobalFunctionsHelper.php
@@ -103,7 +103,7 @@ class GlobalFunctionsHelper
      * @param array $fields
      * @param string|null $delimiter
      * @param string|null $enclosure
-     * @return int
+     * @return int|false
      */
     public function fputcsv($handle, array $fields, $delimiter = null, $enclosure = null)
     {
diff --git a/src/Spout/Reader/CSV/Reader.php b/src/Spout/Reader/CSV/Reader.php
index 942acc5..2597067 100644
--- a/src/Spout/Reader/CSV/Reader.php
+++ b/src/Spout/Reader/CSV/Reader.php
@@ -142,7 +142,7 @@ class Reader extends ReaderAbstract
      */
     protected function closeReader()
     {
-        if ($this->filePointer) {
+        if (is_resource($this->filePointer)) {
             $this->globalFunctionsHelper->fclose($this->filePointer);
         }
 
diff --git a/src/Spout/Reader/CSV/RowIterator.php b/src/Spout/Reader/CSV/RowIterator.php
index 233a14a..dfa2688 100644
--- a/src/Spout/Reader/CSV/RowIterator.php
+++ b/src/Spout/Reader/CSV/RowIterator.php
@@ -21,7 +21,7 @@ class RowIterator implements IteratorInterface
      */
     public const MAX_READ_BYTES_PER_LINE = 0;
 
-    /** @var resource Pointer to the CSV file to read */
+    /** @var resource|null Pointer to the CSV file to read */
     protected $filePointer;
 
     /** @var int Number of read rows */
diff --git a/src/Spout/Reader/ODS/Reader.php b/src/Spout/Reader/ODS/Reader.php
index d07dbb4..00a39bd 100644
--- a/src/Spout/Reader/ODS/Reader.php
+++ b/src/Spout/Reader/ODS/Reader.php
@@ -69,7 +69,7 @@ class Reader extends ReaderAbstract
      */
     protected function closeReader()
     {
-        if ($this->zip) {
+        if ($this->zip !== null) {
             $this->zip->close();
         }
     }
diff --git a/src/Spout/Reader/ReaderAbstract.php b/src/Spout/Reader/ReaderAbstract.php
index 1ef1c20..0c469a1 100644
--- a/src/Spout/Reader/ReaderAbstract.php
+++ b/src/Spout/Reader/ReaderAbstract.php
@@ -233,7 +233,7 @@ abstract class ReaderAbstract implements ReaderInterface
             $this->closeReader();
 
             $sheetIterator = $this->getConcreteSheetIterator();
-            if ($sheetIterator) {
+            if ($sheetIterator !== null) {
                 $sheetIterator->end();
             }
 
diff --git a/src/Spout/Reader/XLSX/Creator/ManagerFactory.php b/src/Spout/Reader/XLSX/Creator/ManagerFactory.php
index d23a53f..e69d429 100644
--- a/src/Spout/Reader/XLSX/Creator/ManagerFactory.php
+++ b/src/Spout/Reader/XLSX/Creator/ManagerFactory.php
@@ -21,7 +21,7 @@ class ManagerFactory
     /** @var CachingStrategyFactory */
     private $cachingStrategyFactory;
 
-    /** @var WorkbookRelationshipsManager */
+    /** @var WorkbookRelationshipsManager|null */
     private $cachedWorkbookRelationshipsManager;
 
     /**
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
index 9afec2f..c36b911 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsCaching/CachingStrategyFactory.php
@@ -84,7 +84,7 @@ class CachingStrategyFactory
 
         $memoryAvailable = $this->getMemoryLimitInKB();
 
-        if ($memoryAvailable === -1) {
+        if ((int) $memoryAvailable === -1) {
             // if cannot get memory limit or if memory limit set as unlimited, don't trust and play safe
             $isInMemoryStrategyUsageSafe = ($sharedStringsUniqueCount < self::MAX_NUM_STRINGS_PER_TEMP_FILE);
         } else {
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
index 85d0103..fcf2a2b 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
@@ -242,7 +242,7 @@ class SharedStringsManager
      */
     public function cleanup()
     {
-        if ($this->cachingStrategy) {
+        if ($this->cachingStrategy !== null) {
             $this->cachingStrategy->clearCache();
         }
     }
diff --git a/src/Spout/Reader/XLSX/Manager/StyleManager.php b/src/Spout/Reader/XLSX/Manager/StyleManager.php
index 71eeead..6435f7c 100644
--- a/src/Spout/Reader/XLSX/Manager/StyleManager.php
+++ b/src/Spout/Reader/XLSX/Manager/StyleManager.php
@@ -60,10 +60,10 @@ class StyleManager
     /** @var array Array containing the IDs of built-in number formats indicating a date */
     protected $builtinNumFmtIdIndicatingDates;
 
-    /** @var array Array containing a mapping NUM_FMT_ID => FORMAT_CODE */
+    /** @var array|null Array containing a mapping NUM_FMT_ID => FORMAT_CODE */
     protected $customNumberFormats;
 
-    /** @var array Array containing a mapping STYLE_ID => [STYLE_ATTRIBUTES] */
+    /** @var array|null Array containing a mapping STYLE_ID => [STYLE_ATTRIBUTES] */
     protected $stylesAttributes;
 
     /** @var array Cache containing a mapping NUM_FMT_ID => IS_DATE_FORMAT. Used to avoid lots of recalculations */
diff --git a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
index 7199f50..e022b34 100644
--- a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
@@ -34,7 +34,7 @@ class WorkbookRelationshipsManager
     /** @var InternalEntityFactory Factory to create entities */
     private $entityFactory;
 
-    /** @var array Cache of the already read workbook relationships: [TYPE] => [FILE_NAME] */
+    /** @var array|null Cache of the already read workbook relationships: [TYPE] => [FILE_NAME] */
     private $cachedWorkbookRelationships;
 
     /**
diff --git a/src/Spout/Reader/XLSX/Reader.php b/src/Spout/Reader/XLSX/Reader.php
index 689f6e2..a031bb7 100644
--- a/src/Spout/Reader/XLSX/Reader.php
+++ b/src/Spout/Reader/XLSX/Reader.php
@@ -119,11 +119,11 @@ class Reader extends ReaderAbstract
      */
     protected function closeReader()
     {
-        if ($this->zip) {
+        if ($this->zip !== null) {
             $this->zip->close();
         }
 
-        if ($this->sharedStringsManager) {
+        if ($this->sharedStringsManager !== null) {
             $this->sharedStringsManager->cleanup();
         }
     }
diff --git a/src/Spout/Writer/Common/Manager/Style/StyleMerger.php b/src/Spout/Writer/Common/Manager/Style/StyleMerger.php
index 806c8d5..cdc45d5 100644
--- a/src/Spout/Writer/Common/Manager/Style/StyleMerger.php
+++ b/src/Spout/Writer/Common/Manager/Style/StyleMerger.php
@@ -88,10 +88,10 @@ class StyleMerger
         if (!$style->hasSetCellAlignment() && $baseStyle->shouldApplyCellAlignment()) {
             $styleToUpdate->setCellAlignment($baseStyle->getCellAlignment());
         }
-        if (!$style->getBorder() && $baseStyle->shouldApplyBorder()) {
+        if ($style->getBorder() === null && $baseStyle->shouldApplyBorder()) {
             $styleToUpdate->setBorder($baseStyle->getBorder());
         }
-        if (!$style->getFormat() && $baseStyle->shouldApplyFormat()) {
+        if ($style->getFormat() === null && $baseStyle->shouldApplyFormat()) {
             $styleToUpdate->setFormat($baseStyle->getFormat());
         }
         if (!$style->shouldApplyBackgroundColor() && $baseStyle->shouldApplyBackgroundColor()) {
diff --git a/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php b/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
index 653778c..9b25e15 100644
--- a/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
+++ b/src/Spout/Writer/Common/Manager/WorkbookManagerAbstract.php
@@ -23,7 +23,7 @@ use Box\Spout\Writer\Exception\WriterException;
  */
 abstract class WorkbookManagerAbstract implements WorkbookManagerInterface
 {
-    /** @var Workbook The workbook to manage */
+    /** @var Workbook|null The workbook to manage */
     protected $workbook;
 
     /** @var OptionsManagerInterface */
@@ -92,7 +92,7 @@ abstract class WorkbookManagerAbstract implements WorkbookManagerInterface
     abstract protected function getWorksheetFilePath(Sheet $sheet);
 
     /**
-     * @return Workbook
+     * @return Workbook|null
      */
     public function getWorkbook()
     {
diff --git a/src/Spout/Writer/Common/Manager/WorkbookManagerInterface.php b/src/Spout/Writer/Common/Manager/WorkbookManagerInterface.php
index aed304a..526163d 100644
--- a/src/Spout/Writer/Common/Manager/WorkbookManagerInterface.php
+++ b/src/Spout/Writer/Common/Manager/WorkbookManagerInterface.php
@@ -17,7 +17,7 @@ use Box\Spout\Writer\Exception\WriterException;
 interface WorkbookManagerInterface
 {
     /**
-     * @return Workbook
+     * @return Workbook|null
      */
     public function getWorkbook();
 
diff --git a/src/Spout/Writer/WriterAbstract.php b/src/Spout/Writer/WriterAbstract.php
index 36a583f..fa43c97 100644
--- a/src/Spout/Writer/WriterAbstract.php
+++ b/src/Spout/Writer/WriterAbstract.php
@@ -169,7 +169,7 @@ abstract class WriterAbstract implements WriterInterface
      */
     protected function throwIfFilePointerIsNotAvailable()
     {
-        if (!$this->filePointer) {
+        if (!is_resource($this->filePointer)) {
             throw new IOException('File pointer has not be opened');
         }
     }
diff --git a/src/Spout/Writer/WriterMultiSheetsAbstract.php b/src/Spout/Writer/WriterMultiSheetsAbstract.php
index 8170b67..18b40bd 100644
--- a/src/Spout/Writer/WriterMultiSheetsAbstract.php
+++ b/src/Spout/Writer/WriterMultiSheetsAbstract.php
@@ -9,7 +9,6 @@ use Box\Spout\Common\Manager\OptionsManagerInterface;
 use Box\Spout\Writer\Common\Creator\ManagerFactoryInterface;
 use Box\Spout\Writer\Common\Entity\Options;
 use Box\Spout\Writer\Common\Entity\Sheet;
-use Box\Spout\Writer\Common\Entity\Worksheet;
 use Box\Spout\Writer\Common\Manager\WorkbookManagerInterface;
 use Box\Spout\Writer\Exception\SheetNotFoundException;
 use Box\Spout\Writer\Exception\WriterAlreadyOpenedException;
@@ -25,7 +24,7 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
     /** @var ManagerFactoryInterface */
     private $managerFactory;
 
-    /** @var WorkbookManagerInterface */
+    /** @var WorkbookManagerInterface|null */
     private $workbookManager;
 
     /**
@@ -66,7 +65,7 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
      */
     protected function openWriter()
     {
-        if (!$this->workbookManager) {
+        if ($this->workbookManager === null) {
             $this->workbookManager = $this->managerFactory->createWorkbookManager($this->optionsManager);
             $this->workbookManager->addNewSheetAndMakeItCurrent();
         }
@@ -85,7 +84,6 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
         $externalSheets = [];
         $worksheets = $this->workbookManager->getWorksheets();
 
-        /** @var Worksheet $worksheet */
         foreach ($worksheets as $worksheet) {
             $externalSheets[] = $worksheet->getExternalSheet();
         }
@@ -143,7 +141,7 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
      */
     protected function throwIfWorkbookIsNotAvailable()
     {
-        if (!$this->workbookManager->getWorkbook()) {
+        if ($this->workbookManager->getWorkbook() === null) {
             throw new WriterNotOpenedException('The writer must be opened before performing this action.');
         }
     }
@@ -162,7 +160,7 @@ abstract class WriterMultiSheetsAbstract extends WriterAbstract
      */
     protected function closeWriter()
     {
-        if ($this->workbookManager) {
+        if ($this->workbookManager !== null) {
             $this->workbookManager->close($this->filePointer);
         }
     }
diff --git a/src/Spout/Writer/XLSX/Creator/ManagerFactory.php b/src/Spout/Writer/XLSX/Creator/ManagerFactory.php
index aa3bcd5..5865268 100644
--- a/src/Spout/Writer/XLSX/Creator/ManagerFactory.php
+++ b/src/Spout/Writer/XLSX/Creator/ManagerFactory.php
@@ -91,8 +91,7 @@ class ManagerFactory implements ManagerFactoryInterface
             $styleMerger,
             $sharedStringsManager,
             $stringsEscaper,
-            $stringsHelper,
-            $this->entityFactory
+            $stringsHelper
         );
     }
 
diff --git a/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php b/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
index da21f7b..9a82cff 100644
--- a/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
+++ b/src/Spout/Writer/XLSX/Manager/SharedStringsManager.php
@@ -59,7 +59,7 @@ EOD;
      */
     protected function throwIfSharedStringsFilePointerIsNotAvailable()
     {
-        if (!$this->sharedStringsFilePointer) {
+        if (!is_resource($this->sharedStringsFilePointer)) {
             throw new IOException('Unable to open shared strings file for writing.');
         }
     }
diff --git a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
index 25bb88d..6025442 100644
--- a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
+++ b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
@@ -10,7 +10,6 @@ use Box\Spout\Common\Exception\IOException;
 use Box\Spout\Common\Helper\Escaper\XLSX as XLSXEscaper;
 use Box\Spout\Common\Helper\StringHelper;
 use Box\Spout\Common\Manager\OptionsManagerInterface;
-use Box\Spout\Writer\Common\Creator\InternalEntityFactory;
 use Box\Spout\Writer\Common\Entity\Options;
 use Box\Spout\Writer\Common\Entity\Worksheet;
 use Box\Spout\Writer\Common\Helper\CellHelper;
@@ -60,9 +59,6 @@ EOD;
     /** @var StringHelper String helper */
     private $stringHelper;
 
-    /** @var InternalEntityFactory Factory to create entities */
-    private $entityFactory;
-
     /**
      * WorksheetManager constructor.
      *
@@ -73,7 +69,6 @@ EOD;
      * @param SharedStringsManager $sharedStringsManager
      * @param XLSXEscaper $stringsEscaper
      * @param StringHelper $stringHelper
-     * @param InternalEntityFactory $entityFactory
      */
     public function __construct(
         OptionsManagerInterface $optionsManager,
@@ -82,8 +77,7 @@ EOD;
         StyleMerger $styleMerger,
         SharedStringsManager $sharedStringsManager,
         XLSXEscaper $stringsEscaper,
-        StringHelper $stringHelper,
-        InternalEntityFactory $entityFactory
+        StringHelper $stringHelper
     ) {
         $this->shouldUseInlineStrings = $optionsManager->getOption(Options::SHOULD_USE_INLINE_STRINGS);
         $this->rowManager = $rowManager;
@@ -92,7 +86,6 @@ EOD;
         $this->sharedStringsManager = $sharedStringsManager;
         $this->stringsEscaper = $stringsEscaper;
         $this->stringHelper = $stringHelper;
-        $this->entityFactory = $entityFactory;
     }
 
     /**
diff --git a/tests/Spout/Writer/CSV/WriterTest.php b/tests/Spout/Writer/CSV/WriterTest.php
index deb3f22..ccf5181 100644
--- a/tests/Spout/Writer/CSV/WriterTest.php
+++ b/tests/Spout/Writer/CSV/WriterTest.php
@@ -193,7 +193,7 @@ class WriterTest extends TestCase
      * @param string $fieldDelimiter
      * @param string $fieldEnclosure
      * @param bool   $shouldAddBOM
-     * @return string|null
+     * @return string
      */
     private function writeToCsvFileAndReturnWrittenContent($allRows, $fileName, $fieldDelimiter = ',', $fieldEnclosure = '"', $shouldAddBOM = true)
     {
diff --git a/tests/Spout/Writer/XLSX/WriterWithStyleTest.php b/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
index 7237039..57859b6 100644
--- a/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
+++ b/tests/Spout/Writer/XLSX/WriterWithStyleTest.php
@@ -475,7 +475,6 @@ class WriterWithStyleTest extends TestCase
             $borderParts = $borderNode->childNodes;
             $ordering = [];
 
-            /** @var \DOMText $part */
             foreach ($borderParts as $part) {
                 if ($part instanceof \DOMElement) {
                     $ordering[] = $part->nodeName;