diff --git a/core/modules/migrate/src/Plugin/migrate/process/Flatten.php b/core/modules/migrate/src/Plugin/migrate/process/Flatten.php
index 324f0b92c8..c38af71134 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/Flatten.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Flatten.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\migrate\Plugin\migrate\process;
 
+use Drupal\migrate\MigrateException;
 use Drupal\migrate\MigrateExecutableInterface;
 use Drupal\migrate\ProcessPluginBase;
 use Drupal\migrate\Row;
@@ -49,6 +50,9 @@ class Flatten extends ProcessPluginBase {
    * For example, [[1, 2, [3, 4]]] becomes [1, 2, 3, 4].
    */
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!is_array($value) && !is_object($value)) {
+      throw new MigrateException(sprintf('Input should be an array or an object, instead it was %s', var_export($value, TRUE)));
+    }
     return iterator_to_array(new \RecursiveIteratorIterator(new \RecursiveArrayIterator($value)), FALSE);
   }
 
diff --git a/core/modules/migrate/tests/src/Unit/process/FlattenTest.php b/core/modules/migrate/tests/src/Unit/process/FlattenTest.php
index 899644f5ae..dc71dc7d94 100644
--- a/core/modules/migrate/tests/src/Unit/process/FlattenTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/FlattenTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\migrate\Unit\process;
 
+use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\migrate\process\Flatten;
 
 /**
@@ -11,13 +12,89 @@
  */
 class FlattenTest extends MigrateProcessTestCase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    $this->plugin = new Flatten([], 'flatten', []);
+    parent::setUp();
+  }
+
   /**
    * Test that various array flatten operations work properly.
+   *
+   * @dataProvider providerTestFlatten
+   */
+  public function testFlatten($value, $expected) {
+    $flattened = $this->plugin->transform($value, $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertSame($expected, $flattened);
+  }
+
+  /**
+   * Provides data for the testFlatten.
+   */
+  public function providerTestFlatten() {
+    $object = (object) [
+      'a' => 'test',
+      'b' => '1.2',
+      'c' => 'NULL',
+    ];
+    return [
+      'array' => [
+        [1, 2, [3, 4, [5]], [], [7, 8]],
+        [1, 2, 3, 4, 5, 7, 8],
+      ],
+      'object' => [
+        $object,
+        ['test', '1.2', 'NULL'],
+      ],
+    ];
+  }
+
+  /**
+   * Tests that Flatten throws a MigrateException.
+   *
+   * @dataProvider providerTestFlattenInvalid
+   */
+  public function testFlattenInvalid($value) {
+    $this->expectException(MigrateException::class);
+    $this->expectExceptionMessage(sprintf('Input should be an array or an object, instead it was %s', print_r($value, TRUE)));
+    $this->plugin->transform($value, $this->migrateExecutable, $this->row, 'destination_property');
+  }
+
+  /**
+   * Provides data for the testFlattenInvalid.
    */
-  public function testFlatten() {
-    $plugin = new Flatten([], 'flatten', []);
-    $flattened = $plugin->transform([1, 2, [3, 4, [5]], [], [7, 8]], $this->migrateExecutable, $this->row, 'destination_property');
-    $this->assertSame([1, 2, 3, 4, 5, 7, 8], $flattened);
+  public function providerTestFlattenInvalid() {
+    $xml_str = <<<XML
+<xml version='1.0'?>
+<authors>
+ <name>Tamora Pierce</name>
+</authors>
+XML;
+    return [
+      'empty string' => [
+        '',
+      ],
+      'string' => [
+        'Kate Sheppard',
+      ],
+      'integer' => [
+        1,
+      ],
+      'float' => [
+        1.2,
+      ],
+      'NULL' => [
+        NULL,
+      ],
+      'boolean' => [
+        TRUE,
+      ],
+      'xml' => [
+        $xml_str,
+      ],
+    ];
   }
 
 }