diff --git a/system/BaseModel.php b/system/BaseModel.php
index 06af2ded411f..ee3d1a859955 100644
--- a/system/BaseModel.php
+++ b/system/BaseModel.php
@@ -1099,6 +1099,21 @@ public function updateBatch(?array $set = null, ?string $index = null, int $batc
     {
         if (is_array($set)) {
             foreach ($set as &$row) {
+                // Save the index value from the original row because
+                // transformDataToArray() may strip it when updateOnlyChanged
+                // is true.
+                $updateIndex = null;
+
+                if ($this->updateOnlyChanged) {
+                    if (is_array($row)) {
+                        $updateIndex = $row[$index] ?? null;
+                    } elseif ($row instanceof Entity) {
+                        $updateIndex = $row->toRawArray()[$index] ?? null;
+                    } elseif (is_object($row)) {
+                        $updateIndex = $row->{$index} ?? null;
+                    }
+                }
+
                 $row = $this->transformDataToArray($row, 'update');
 
                 // Validate data before saving.
@@ -1106,8 +1121,13 @@ public function updateBatch(?array $set = null, ?string $index = null, int $batc
                     return false;
                 }
 
-                // Save updateIndex for later
-                $updateIndex = $row[$index] ?? null;
+                // When updateOnlyChanged is true, restore the pre-extracted
+                // index into $row. Otherwise read it from the transformed row.
+                if ($updateIndex !== null) {
+                    $row[$index] = $updateIndex;
+                } else {
+                    $updateIndex = $row[$index] ?? null;
+                }
 
                 if ($updateIndex === null) {
                     throw new InvalidArgumentException(
diff --git a/tests/system/Models/UpdateModelTest.php b/tests/system/Models/UpdateModelTest.php
index 94f8df12ea73..2495834ba978 100644
--- a/tests/system/Models/UpdateModelTest.php
+++ b/tests/system/Models/UpdateModelTest.php
@@ -265,6 +265,43 @@ public function testUpdateBatchWithEntity(): void
         $this->assertSame(2, $model->updateBatch([$entity1, $entity2], 'id'));
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/issues/9943
+     */
+    public function testUpdateBatchWithEntityAndUpdateOnlyChanged(): void
+    {
+        $entity1          = new User();
+        $entity1->id      = 1;
+        $entity1->name    = 'Derek Jones';
+        $entity1->email   = 'derek@world.com';
+        $entity1->country = 'US';
+        $entity1->syncOriginal();
+        $entity1->country = 'Greece';
+
+        $entity2          = new User();
+        $entity2->id      = 4;
+        $entity2->name    = 'Chris Martin';
+        $entity2->email   = 'chris@world.com';
+        $entity2->country = 'UK';
+        $entity2->syncOriginal();
+        $entity2->country = 'Finland';
+
+        // updateOnlyChanged is true by default. The index field 'id' is
+        // unchanged but must be preserved for the batch WHERE clause.
+        $model = $this->createModel(UserModel::class);
+        $this->assertTrue($this->getPrivateProperty($model, 'updateOnlyChanged'));
+        $this->assertSame(2, $model->updateBatch([$entity1, $entity2], 'id'));
+
+        $this->seeInDatabase('user', [
+            'name'    => 'Derek Jones',
+            'country' => 'Greece',
+        ]);
+        $this->seeInDatabase('user', [
+            'name'    => 'Chris Martin',
+            'country' => 'Finland',
+        ]);
+    }
+
     public function testUpdateNoPrimaryKey(): void
     {
         $this->db->table('secondary')->insert([
diff --git a/user_guide_src/source/changelogs/v4.7.1.rst b/user_guide_src/source/changelogs/v4.7.1.rst
index 8a94a731a413..a6152e840301 100644
--- a/user_guide_src/source/changelogs/v4.7.1.rst
+++ b/user_guide_src/source/changelogs/v4.7.1.rst
@@ -33,6 +33,7 @@ Bugs Fixed
 **********
 
 - **ContentSecurityPolicy:** Fixed a bug where ``generateNonces()`` produces corrupted JSON responses by replacing CSP nonce placeholders with unescaped double quotes. The method now automatically JSON-escapes nonce attributes when the response Content-Type is JSON.
+- **Model:** Fixed a bug where ``BaseModel::updateBatch()`` threw an exception when ``updateOnlyChanged`` was ``true`` and the index field value did not change.
 - **Session:** Fixed a bug in ``MemcachedHandler`` where the constructor incorrectly threw an exception when ``savePath`` was not empty.
 
 See the repo's
diff --git a/utils/phpstan-baseline/missingType.property.neon b/utils/phpstan-baseline/missingType.property.neon
index fdf4057fee76..2083602a9d54 100644
--- a/utils/phpstan-baseline/missingType.property.neon
+++ b/utils/phpstan-baseline/missingType.property.neon
@@ -468,42 +468,42 @@ parameters:
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$_options has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$_options has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$country has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$country has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$created_at has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$created_at has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$deleted has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$deleted has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$email has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$email has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$id has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$id has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$name has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$name has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php
 
         -
-            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:364\:\:\$updated_at has no type specified\.$#'
+            message: '#^Property CodeIgniter\\Entity\\Entity@anonymous/tests/system/Models/UpdateModelTest\.php\:401\:\:\$updated_at has no type specified\.$#'
             count: 1
             path: ../../tests/system/Models/UpdateModelTest.php