diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 4f8b839..c67f9d7 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -5,6 +5,7 @@ Yii Framework 2 Change Log
 ------------------------
 
 - Bug #19138: Allow digits in language code (ntesic)
+- Bug #19148: Fix undefined array key errors in `yii\db\ActiveRelationTrait` (stevekr)
 - Bug #19041: Fix PHP 8.1 issues (longthanhtran, samdark, pamparam83, sartor, githubjeka)
 
 
diff --git a/framework/db/ActiveRelationTrait.php b/framework/db/ActiveRelationTrait.php
index 0e0b5f2..24fa0af 100644
--- a/framework/db/ActiveRelationTrait.php
+++ b/framework/db/ActiveRelationTrait.php
@@ -289,7 +289,12 @@ trait ActiveRelationTrait
             $link = array_values($deepViaQuery->link);
         }
         foreach ($primaryModels as $i => $primaryModel) {
-            if ($this->multiple && count($link) === 1 && is_array($keys = $primaryModel[reset($link)])) {
+            $keys = null;
+            if ($this->multiple && count($link) === 1) {
+                $primaryModelKey = reset($link);
+                $keys = isset($primaryModel[$primaryModelKey]) ? $primaryModel[$primaryModelKey] : null;
+            }
+            if (is_array($keys)) {
                 $value = [];
                 foreach ($keys as $key) {
                     $key = $this->normalizeModelKey($key);
@@ -523,7 +528,8 @@ trait ActiveRelationTrait
             // single key
             $attribute = reset($this->link);
             foreach ($models as $model) {
-                if (($value = $model[$attribute]) !== null) {
+                $value = isset($model[$attribute]) ? $model[$attribute] : null;
+                if ($value !== null) {
                     if (is_array($value)) {
                         $values = array_merge($values, $value);
                     } elseif ($value instanceof ArrayExpression && $value->getDimension() === 1) {
@@ -574,13 +580,15 @@ trait ActiveRelationTrait
     /**
      * @param ActiveRecordInterface|array $model
      * @param array $attributes
-     * @return string
+     * @return string|false
      */
     private function getModelKey($model, $attributes)
     {
         $key = [];
         foreach ($attributes as $attribute) {
-            $key[] = $this->normalizeModelKey($model[$attribute]);
+            if (isset($model[$attribute])) {
+                $key[] = $this->normalizeModelKey($model[$attribute]);
+            }
         }
         if (count($key) > 1) {
             return serialize($key);
diff --git a/tests/framework/db/ActiveRecordTest.php b/tests/framework/db/ActiveRecordTest.php
index 0088a90..3881be6 100644
--- a/tests/framework/db/ActiveRecordTest.php
+++ b/tests/framework/db/ActiveRecordTest.php
@@ -1361,7 +1361,7 @@ abstract class ActiveRecordTest extends DatabaseTestCase
         $model->char_col2 = 'not something';
         $model->loadDefaultValues(false);
         $this->assertEquals('something', $model->char_col2);
-        
+
         // Cropped model with 2 attributes/columns
         $model = new CroppedType();
         $model->loadDefaultValues();
@@ -1529,6 +1529,29 @@ abstract class ActiveRecordTest extends DatabaseTestCase
         ];
         $this->assertEquals($expected, $aggregation);
 
+        // tests with single pk asArray with eager loading
+        $aggregation = Customer::find()
+            ->select(['{{customer}}.[[status]]', 'SUM({{order}}.[[total]]) AS [[sumtotal]]'])
+            ->joinWith('ordersPlain')
+            ->groupBy('{{customer}}.[[status]]')
+            ->orderBy('status')
+            ->asArray()
+            ->all();
+
+        $expected = [
+            [
+                'status' => 1,
+                'sumtotal' => 183,
+                'ordersPlain' => [],
+            ],
+            [
+                'status' => 2,
+                'sumtotal' => 0,
+                'ordersPlain' => [],
+            ],
+        ];
+        $this->assertEquals($expected, $aggregation);
+
         // tests with single pk with Models
         $aggregation = Customer::find()
             ->select(['{{customer}}.[[status]]', 'SUM({{order}}.[[total]]) AS [[sumTotal]]'])