diff --git a/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php b/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
index 2c9bb2de..b2b0e669 100644
--- a/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
+++ b/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
@@ -27,7 +27,7 @@ abstract class DatabaseAbstract
      */
     protected static function fieldExtract(array $database, $field): ?int
     {
-        $field = strtoupper(Functions::flattenSingleValue($field));
+        $field = strtoupper(Functions::flattenSingleValue($field ?? ''));
         if ($field === '') {
             return null;
         }
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
index de767fca..d8e88a21 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
@@ -37,7 +37,7 @@ class DateValue
     {
         $dti = new DateTimeImmutable();
         $baseYear = SharedDateHelper::getExcelCalendar();
-        $dateValue = trim(Functions::flattenSingleValue($dateValue), '"');
+        $dateValue = trim(Functions::flattenSingleValue($dateValue ?? ''), '"');
         //    Strip any ordinals because they're allowed in Excel (English only)
         $dateValue = preg_replace('/(\d)(st|nd|rd|th)([ -\/])/Ui', '$1$3', $dateValue) ?? '';
         //    Convert separators (/ . or space) to hyphens (should also handle dot used for ordinals in some countries, e.g. Denmark, Germany)
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/TimeValue.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/TimeValue.php
index 73b7ba91..2a294344 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/TimeValue.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/TimeValue.php
@@ -31,7 +31,7 @@ class TimeValue
      */
     public static function fromString($timeValue)
     {
-        $timeValue = trim(Functions::flattenSingleValue($timeValue), '"');
+        $timeValue = trim(Functions::flattenSingleValue($timeValue ?? ''), '"');
         $timeValue = str_replace(['/', '.'], '-', $timeValue);
 
         $arraySplit = preg_split('/[\/:\-\s]/', $timeValue) ?: [];
diff --git a/src/PhpSpreadsheet/Calculation/TextData/Extract.php b/src/PhpSpreadsheet/Calculation/TextData/Extract.php
index 2f994858..015fabfb 100644
--- a/src/PhpSpreadsheet/Calculation/TextData/Extract.php
+++ b/src/PhpSpreadsheet/Calculation/TextData/Extract.php
@@ -26,7 +26,7 @@ class Extract
             $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
         }
 
-        return mb_substr($value, 0, $chars, 'UTF-8');
+        return mb_substr($value ?? '', 0, $chars, 'UTF-8');
     }
 
     /**
@@ -50,7 +50,7 @@ class Extract
             $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
         }
 
-        return mb_substr($value, --$start, $chars, 'UTF-8');
+        return mb_substr($value ?? '', --$start, $chars, 'UTF-8');
     }
 
     /**
@@ -72,6 +72,6 @@ class Extract
             $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
         }
 
-        return mb_substr($value, mb_strlen($value, 'UTF-8') - $chars, $chars, 'UTF-8');
+        return mb_substr($value ?? '', mb_strlen($value ?? '', 'UTF-8') - $chars, $chars, 'UTF-8');
     }
 }
diff --git a/src/PhpSpreadsheet/Calculation/TextData/Text.php b/src/PhpSpreadsheet/Calculation/TextData/Text.php
index 6e408891..f3da4bb3 100644
--- a/src/PhpSpreadsheet/Calculation/TextData/Text.php
+++ b/src/PhpSpreadsheet/Calculation/TextData/Text.php
@@ -20,7 +20,7 @@ class Text
             $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
         }
 
-        return mb_strlen($value, 'UTF-8');
+        return mb_strlen($value ?? '', 'UTF-8');
     }
 
     /**
diff --git a/src/PhpSpreadsheet/Reader/Html.php b/src/PhpSpreadsheet/Reader/Html.php
index 6e6155c2..ced65e97 100644
--- a/src/PhpSpreadsheet/Reader/Html.php
+++ b/src/PhpSpreadsheet/Reader/Html.php
@@ -914,7 +914,7 @@ class Html extends BaseReader
      */
     public function getStyleColor($value)
     {
-        if (strpos($value, '#') === 0) {
+        if (strpos($value ?? '', '#') === 0) {
             return substr($value, 1);
         }
 
diff --git a/src/PhpSpreadsheet/Reader/Xml/PageSettings.php b/src/PhpSpreadsheet/Reader/Xml/PageSettings.php
index e56ac331..62e1e385 100644
--- a/src/PhpSpreadsheet/Reader/Xml/PageSettings.php
+++ b/src/PhpSpreadsheet/Reader/Xml/PageSettings.php
@@ -115,7 +115,7 @@ class PageSettings
 
     private function setLayout(stdClass $printDefaults, SimpleXMLElement $pageSetupAttributes): void
     {
-        $printDefaults->orientation = (string) strtolower($pageSetupAttributes->Orientation) ?: PageSetup::ORIENTATION_PORTRAIT;
+        $printDefaults->orientation = (string) strtolower($pageSetupAttributes->Orientation ?? '') ?: PageSetup::ORIENTATION_PORTRAIT;
         $printDefaults->horizontalCentered = (bool) $pageSetupAttributes->CenterHorizontal ?: false;
         $printDefaults->verticalCentered = (bool) $pageSetupAttributes->CenterVertical ?: false;
     }
diff --git a/src/PhpSpreadsheet/Shared/StringHelper.php b/src/PhpSpreadsheet/Shared/StringHelper.php
index e85ce55d..0e8eb8a5 100644
--- a/src/PhpSpreadsheet/Shared/StringHelper.php
+++ b/src/PhpSpreadsheet/Shared/StringHelper.php
@@ -502,7 +502,7 @@ class StringHelper
      */
     public static function strToLower($pValue)
     {
-        return mb_convert_case($pValue, MB_CASE_LOWER, 'UTF-8');
+        return mb_convert_case($pValue ?? '', MB_CASE_LOWER, 'UTF-8');
     }
 
     /**
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index 38dead4f..493fd07e 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -1256,7 +1256,7 @@ class Worksheet extends WriterPart
             $objWriter,
             $this->getParentWriter()->getOffice2003Compatibility() === false,
             'v',
-            ($this->getParentWriter()->getPreCalculateFormulas() && !is_array($calculatedValue) && substr($calculatedValue, 0, 1) !== '#')
+            ($this->getParentWriter()->getPreCalculateFormulas() && !is_array($calculatedValue) && substr($calculatedValue ?? '', 0, 1) !== '#')
                 ? StringHelper::formatNumber($calculatedValue) : '0'
         );
     }
diff --git a/tests/PhpSpreadsheetTests/Calculation/FunctionsTest.php b/tests/PhpSpreadsheetTests/Calculation/FunctionsTest.php
index 4e8335f1..ea629183 100644
--- a/tests/PhpSpreadsheetTests/Calculation/FunctionsTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/FunctionsTest.php
@@ -341,7 +341,7 @@ class FunctionsTest extends TestCase
                 ->disableOriginalConstructor()
                 ->getMock();
             $remoteCell->method('isFormula')
-                ->willReturn(substr($value, 0, 1) == '=');
+                ->willReturn(substr($value ?? '', 0, 1) == '=');
 
             $remoteSheet = $this->getMockBuilder(Worksheet::class)
                 ->disableOriginalConstructor()