diff -u b/core/modules/filter/migration_templates/d6_filter_format.yml b/core/modules/filter/migration_templates/d6_filter_format.yml
--- b/core/modules/filter/migration_templates/d6_filter_format.yml
+++ b/core/modules/filter/migration_templates/d6_filter_format.yml
@@ -16,12 +16,16 @@
     key: '@id'
     process:
       id:
-        # Because the bypass flag is not set, the row will be skipped
-        # entirely if the filter ID cannot be determined.
+        # If the filter ID cannot be mapped, it will be passed through
+        # unchanged because the bypass flag is set. The filter_id plugin
+        # will flatten the input value and default it to filter_null (the
+        # fallback filter plugin ID) if the flattened input value is not
+        # a valid plugin ID.
         plugin: filter_id
         source:
           - module
           - delta
+        bypass: true
         map:
           filter:
             - filter_html
diff -u b/core/modules/filter/src/Plugin/migrate/process/FilterID.php b/core/modules/filter/src/Plugin/migrate/process/FilterID.php
--- b/core/modules/filter/src/Plugin/migrate/process/FilterID.php
+++ b/core/modules/filter/src/Plugin/migrate/process/FilterID.php
@@ -4,6 +4,7 @@
 
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\StringTranslation\TranslationInterface;
 use Drupal\migrate\MigrateExecutableInterface;
 use Drupal\migrate\Plugin\migrate\process\StaticMap;
 use Drupal\migrate\Plugin\MigrationInterface;
@@ -35,10 +36,13 @@
    *   The plugin definition.
    * @param \Drupal\Component\Plugin\PluginManagerInterface $filter_manager
    *   The filter plugin manager.
+   * @param TranslationInterface $translator
+   *   (optional) The string translation service.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, PluginManagerInterface $filter_manager) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, PluginManagerInterface $filter_manager, TranslationInterface $translator = NULL) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->filterManager = $filter_manager;
+    $this->stringTranslation = $translator;
   }
 
   /**
@@ -49,7 +53,8 @@
       $configuration,
       $plugin_id,
       $plugin_definition,
-      $container->get('plugin.manager.filter')
+      $container->get('plugin.manager.filter'),
+      $container->get('string_translation')
     );
   }
 
@@ -59,20 +64,24 @@
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
     $plugin_id = parent::transform($value, $migrate_executable, $row, $destination_property);
 
+    // If the static map is bypassed on failure, the returned plugin ID will be
+    // an array if $value was. Plugin IDs cannot be arrays, so flatten it before
+    // passing it into the filter manager.
+    if (is_array($plugin_id)) {
+      $plugin_id = implode(':', $plugin_id);
+    }
+
     if ($this->filterManager->hasDefinition($plugin_id)) {
       return $plugin_id;
     }
     else {
       $fallback = $this->filterManager->getFallbackPluginId($plugin_id);
 
-      if (is_array($value)) {
-        $value = implode(', ', $value);
-      }
-      $message = $this->t('Filter @id could not be mapped to an existing filter plugin; defaulting to @fallback.', [
-        '@id' => $value,
+      $message = $this->t('Filter @plugin_id could not be mapped to an existing filter plugin; defaulting to @fallback.', [
+        '@plugin_id' => $plugin_id,
         '@fallback' => $fallback,
       ]);
-      $migrate_executable->saveMessage($message, MigrationInterface::MESSAGE_WARNING);
+      $migrate_executable->saveMessage((string) $message, MigrationInterface::MESSAGE_WARNING);
 
       return $fallback;
     }
reverted:
--- b/core/modules/filter/tests/src/Unit/Plugin/migrate/process/FilterIdTest.php
+++ /dev/null
@@ -1,146 +0,0 @@
-<?php
-
-namespace Drupal\Tests\filter\Unit\Plugin\migrate\process;
-
-use Drupal\Core\DependencyInjection\ContainerBuilder;
-use Drupal\Core\StringTranslation\TranslationInterface;
-use Drupal\filter\FilterPluginManager;
-use Drupal\filter\Plugin\migrate\process\FilterID;
-use Drupal\migrate\MigrateExecutableInterface;
-use Drupal\migrate\Row;
-use Drupal\Tests\UnitTestCase;
-use Prophecy\Argument;
-use Prophecy\Prediction\CallPrediction;
-use Prophecy\Prediction\NoCallsPrediction;
-use Prophecy\Prediction\PredictionInterface;
-
-/**
- * Unit tests of the filter_id plugin.
- *
- * @coversDefaultClass \Drupal\filter\Plugin\migrate\process\FilterID
- * @group filter
- */
-class FilterIdTest extends UnitTestCase {
-
-  /**
-   * The FilterPluginManager prophecy.
-   *
-   * @var \Prophecy\Prophecy\ObjectProphecy|FilterPluginManager
-   */
-  protected $filterManager;
-
-  /**
-   * The MigrateExecutable prophecy.
-   *
-   * @var \Prophecy\Prophecy\ObjectProphecy|MigrateExecutableInterface
-   */
-  protected $executable;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    parent::setUp();
-
-    $this->filterManager = $this->prophesize(FilterPluginManager::class);
-    $this->executable = $this->prophesize(MigrateExecutableInterface::class);
-
-    $this->filterManager
-      ->hasDefinition('filter_html')
-      ->willReturn(TRUE);
-
-    $this->filterManager
-      ->hasDefinition('php_code')
-      ->willReturn(FALSE);
-
-    $this->filterManager
-      ->getFallbackPluginId(Argument::type('string'))
-      ->willReturn('filter_null');
-
-    // Mock the string translation service, since the plugin will translate
-    // any message it saves.
-    $translator = $this->prophesize(TranslationInterface::class);
-
-    $translator
-      ->translate(
-        Argument::type('string')
-      )
-      ->willReturnArgument(0);
-
-    $translator
-      ->translate(
-        Argument::type('string'),
-        Argument::type('array')
-      )
-      ->willReturnArgument(0);
-
-    $container = new ContainerBuilder();
-    $container->set('string_translation', $translator->reveal());
-
-    \Drupal::setContainer($container);
-  }
-
-  /**
-   * Tests the filter_id plugin.
-   *
-   * @param mixed $value
-   *   The input value to the plugin.
-   * @param string $expected_value
-   *   The output value expected from the plugin.
-   * @param PredictionInterface $message_saved
-   *   A prediction for if and how the MigrateExecutable's saveMessage() method
-   *   will be called.
-   *
-   * @dataProvider testProvider
-   *
-   * @covers ::transform
-   */
-  public function test($value, $expected_value, PredictionInterface $message_saved) {
-    $plugin = new FilterID(
-      [
-        'bypass' => TRUE,
-        'map' => [
-          'foo' => 'filter_html',
-          'baz' => 'php_code',
-        ],
-      ],
-      'filter_id',
-      [],
-      $this->filterManager->reveal()
-    );
-
-    $this->executable
-      ->saveMessage(
-        Argument::any(),
-        Argument::type('integer')
-      )
-      ->should($message_saved);
-
-    $row = new Row([], []);
-    $output_value = $plugin->transform($value, $this->executable->reveal(), $row, 'foo');
-
-    $this->assertSame($expected_value, $output_value);
-    $this->executable->checkProphecyMethodsPredictions();
-  }
-
-  /**
-   * The test data provider.
-   *
-   * @return array
-   */
-  public function testProvider() {
-    return [
-      // The filter ID is mapped, and the plugin exists.
-      ['foo', 'filter_html', new NoCallsPrediction],
-      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
-      // it bypasses the static map) and the plugin exists.
-      ['filter_html', 'filter_html', new NoCallsPrediction],
-      // The filter ID is mapped, but the plugin does not exist.
-      ['baz', 'filter_null', new CallPrediction],
-      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
-      // it bypasses the static map) but the plugin does not exist.
-      ['php_code', 'filter_null', new CallPrediction],
-    ];
-  }
-
-}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/filter/tests/src/Kernel/Plugin/migrate/process/FilterIdTest.php
@@ -0,0 +1,119 @@
+<?php
+
+namespace Drupal\Tests\filter\Kernel\Plugin\migrate\process;
+
+use Drupal\filter\Plugin\migrate\process\FilterID;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\migrate\Row;
+
+/**
+ * Unit tests of the filter_id plugin.
+ *
+ * @coversDefaultClass \Drupal\filter\Plugin\migrate\process\FilterID
+ * @group filter
+ */
+class FilterIdTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['filter'];
+
+  /**
+   * The mocked MigrateExecutable.
+   *
+   * @var MigrateExecutableInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $executable;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->executable = $this->getMock(MigrateExecutableInterface::class);
+  }
+
+  /**
+   * Tests the filter_id plugin.
+   *
+   * @param mixed $value
+   *   The input value to the plugin.
+   * @param string $expected_value
+   *   The output value expected from the plugin.
+   * @param string $invalid_id
+   *   (optional) The invalid plugin ID which is expected to be logged by the
+   *   MigrateExecutable object.
+   *
+   * @dataProvider testProvider
+   *
+   * @covers ::transform
+   */
+  public function test($value, $expected_value, $invalid_id = NULL) {
+    $configuration = [
+      'bypass' => TRUE,
+      'map' => [
+        'foo' => 'filter_html',
+        'baz' => 'php_code',
+      ],
+    ];
+    $plugin = FilterID::create($this->container, $configuration, 'filter_id', []);
+
+    if (isset($invalid_id)) {
+      $this->executable
+        ->expects($this->exactly(1))
+        ->method('saveMessage')
+        ->with(
+          'Filter ' . $invalid_id . ' could not be mapped to an existing filter plugin; defaulting to filter_null.',
+          MigrationInterface::MESSAGE_WARNING
+        );
+    }
+
+    $row = new Row([], []);
+    $output_value = $plugin->transform($value, $this->executable, $row, 'foo');
+
+    $this->assertSame($expected_value, $output_value);
+  }
+
+  /**
+   * The test data provider.
+   *
+   * @return array
+   */
+  public function testProvider() {
+    return [
+      // The filter ID is mapped, and the plugin exists.
+      [
+        'foo',
+        'filter_html',
+      ],
+      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
+      // it bypasses the static map) and the plugin exists.
+      [
+        'filter_html',
+        'filter_html',
+      ],
+      // The filter ID is mapped, but the plugin does not exist.
+      [
+        'baz',
+        'filter_null',
+        'php_code',
+      ],
+      // The filter ID isn't mapped, but it's unchanged from the source (i.e.,
+      // it bypasses the static map) but the plugin does not exist.
+      [
+        'php_code',
+        'filter_null',
+        'php_code',
+      ],
+      [
+        ['filter', 1],
+        'filter_null',
+        'filter:1',
+      ],
+    ];
+  }
+
+}