diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2386913d3e2..ff15efe80d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 - Fixed a bug where element chips within thumbnail views weren’t getting light gray backgrounds. ([#15649](https://github.com/craftcms/cms/issues/15649))
 - Fixed a bug where Link fields didn’t fully support inline editing. ([#15653](https://github.com/craftcms/cms/issues/15653))
 - Fixed the loading spinner styling on element indexes. ([#15634](https://github.com/craftcms/cms/issues/15634))
+- Fixed an error that could occur when saving an element. ([#15656](https://github.com/craftcms/cms/issues/15656))
 
 ## 5.4.0.1 - 2024-09-03
 
diff --git a/src/services/Elements.php b/src/services/Elements.php
index d5b2adee703..8ad0edc5a21 100644
--- a/src/services/Elements.php
+++ b/src/services/Elements.php
@@ -57,6 +57,7 @@
 use craft\helpers\Db;
 use craft\helpers\ElementHelper;
 use craft\helpers\Html;
+use craft\helpers\Json;
 use craft\helpers\Queue;
 use craft\helpers\StringHelper;
 use craft\helpers\UrlHelper;
@@ -3739,7 +3740,11 @@ private function _saveElementInternal(
 
                 // if we're supposed to save all the content
                 if ($saveContent || !empty($dirtyFields)) {
-                    $oldContent = $siteSettingsRecord->content; // we'll need that if we're not saving all the content
+                    $oldContent = $siteSettingsRecord->content ?? []; // we'll need that if we're not saving all the content
+                    if (is_string($oldContent)) {
+                        $oldContent = $oldContent !== '' ? Json::decode($oldContent) : [];
+                    }
+
                     $content = [];
                     if ($fieldLayout) {
                         foreach ($fieldLayout->getCustomFields() as $field) {