diff --git a/typo3/sysext/backend/Classes/Form/Element/AbstractFormElement.php b/typo3/sysext/backend/Classes/Form/Element/AbstractFormElement.php
index e48be6b947a0501438693b9de3797186a0888068..27866c5276bb7fb6319fc24e161452a6796f67b6 100644
--- a/typo3/sysext/backend/Classes/Form/Element/AbstractFormElement.php
+++ b/typo3/sysext/backend/Classes/Form/Element/AbstractFormElement.php
@@ -296,7 +296,8 @@ abstract class AbstractFormElement extends AbstractNode
             default:
                 // Do nothing e.g. when $format === ''
         }
-        return $itemValue;
+        // Make sure we have a string in the end. $itemValue could be null, for instance.
+        return (string)$itemValue;
     }
 
     /**
diff --git a/typo3/sysext/backend/Classes/Form/Element/InputDateTimeElement.php b/typo3/sysext/backend/Classes/Form/Element/InputDateTimeElement.php
index 19a17976d412d44b8f6e4f47a43e8e6044e21e77..46d19a4303d932c9349761efd4e29c73e4c378a3 100644
--- a/typo3/sysext/backend/Classes/Form/Element/InputDateTimeElement.php
+++ b/typo3/sysext/backend/Classes/Form/Element/InputDateTimeElement.php
@@ -198,7 +198,7 @@ class InputDateTimeElement extends AbstractFormElement
         $expansionHtml[] =      '<div class="form-wizards-element">';
         $expansionHtml[] =          '<div class="input-group">';
         $expansionHtml[] =              '<input type="text" ' . GeneralUtility::implodeAttributes($attributes, true) . ' />';
-        $expansionHtml[] =              '<input type="hidden" name="' . $parameterArray['itemFormElName'] . '" value="' . htmlspecialchars($itemValue) . '" />';
+        $expansionHtml[] =              '<input type="hidden" name="' . $parameterArray['itemFormElName'] . '" value="' . htmlspecialchars((string)$itemValue) . '" />';
         $expansionHtml[] =              '<button class="btn btn-default" type="button" data-global-event="click" data-action-focus="#' . $attributes['id'] . '">';
         $expansionHtml[] =                  $this->iconFactory->getIcon('actions-edit-pick-date', Icon::SIZE_SMALL)->render();
         $expansionHtml[] =              '</button>';
diff --git a/typo3/sysext/backend/Tests/Unit/Form/Element/AbstractFormElementTest.php b/typo3/sysext/backend/Tests/Unit/Form/Element/AbstractFormElementTest.php
index b9174ee599027eeaae5409e1eec9fac91cd5ede1..3408dfd1b16185f8a39c6fba0cfb3d3a4cbf60b8 100644
--- a/typo3/sysext/backend/Tests/Unit/Form/Element/AbstractFormElementTest.php
+++ b/typo3/sysext/backend/Tests/Unit/Form/Element/AbstractFormElementTest.php
@@ -279,6 +279,6 @@ class AbstractFormElementTest extends UnitTestCase
         $result = $subject->_call('formatValue', $config['format'], $itemValue, $config['format.'] ?? []);
         date_default_timezone_set($timezoneBackup);
 
-        self::assertEquals($expectedResult, $result);
+        self::assertSame($expectedResult, $result);
     }
 }