diff --git a/core/modules/filter/src/Plugin/migrate/process/FilterSettings.php b/core/modules/filter/src/Plugin/migrate/process/FilterSettings.php
index 235042609c3b6db5397a1d3780b62ef6696cfb1c..eb0da328d0675bde4c0f3a1df0f9f8237216d77d 100644
--- a/core/modules/filter/src/Plugin/migrate/process/FilterSettings.php
+++ b/core/modules/filter/src/Plugin/migrate/process/FilterSettings.php
@@ -42,7 +42,9 @@ class FilterSettings extends ProcessPluginBase {
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
     // Only the filter_html filter's settings have a changed format.
     if ($row->getDestinationProperty('id') === 'filter_html') {
-      $value['allowed_html'] = str_replace(array_keys($this->allowedHtmlDefaultAttributes), array_values($this->allowedHtmlDefaultAttributes), $value['allowed_html']);
+      if (!empty($value['allowed_html'])) {
+        $value['allowed_html'] = str_replace(array_keys($this->allowedHtmlDefaultAttributes), array_values($this->allowedHtmlDefaultAttributes), $value['allowed_html']);
+      }
     }
     return $value;
   }
diff --git a/core/modules/filter/tests/src/Kernel/Plugin/migrate/process/FilterSettingsTest.php b/core/modules/filter/tests/src/Kernel/Plugin/migrate/process/FilterSettingsTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..42502110775c5e2abbf9446e59a93e1f47059d29
--- /dev/null
+++ b/core/modules/filter/tests/src/Kernel/Plugin/migrate/process/FilterSettingsTest.php
@@ -0,0 +1,102 @@
+<?php
+
+namespace Drupal\Tests\filter\Kernel\Plugin\migrate\process;
+
+use Drupal\filter\Plugin\migrate\process\FilterSettings;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\migrate\Row;
+use Drupal\Tests\migrate\Unit\MigrateTestCase;
+
+/**
+ * Unit tests of the filter_settings plugin.
+ *
+ * @coversDefaultClass \Drupal\filter\Plugin\migrate\process\FilterSettings
+ * @group filter
+ */
+class FilterSettingsTest extends MigrateTestCase {
+
+  /**
+   * Tests transformation of filter settings.
+   *
+   * @dataProvider dataProvider
+   * @covers ::transform
+   */
+  public function testTransform($value, $destination_id, $expected_value) {
+    $migration = $this->getMock(MigrationInterface::class);
+    $plugin = new FilterSettings([], 'filter_settings', [], $migration);
+
+    $executable = $this->getMock(MigrateExecutableInterface::class);
+    $row = $this->getMockBuilder(Row::class)
+      ->disableOriginalConstructor()
+      ->getMock();
+
+    $row->expects($this->atLeastOnce())
+      ->method('getDestinationProperty')
+      ->willReturn($destination_id);
+
+    $output_value = $plugin->transform($value, $executable, $row, 'foo');
+    $this->assertSame($expected_value, $output_value);
+  }
+
+  /**
+   * The test data provider.
+   *
+   * @return array
+   */
+  public function dataProvider() {
+    return [
+      // Tests that the transformed value is identical to the input value when
+      // destination is not the filter_html.
+
+      // Test with an empty source array.
+      [
+        [],
+        'any_filter',
+        [],
+      ],
+      // Test with a source string.
+      [
+        'a string',
+        'any_filter',
+        'a string',
+      ],
+      // Test with a source filter array.
+      [
+        [
+          'allowed_html' => '<a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>',
+        ],
+        'any_filter',
+        [
+          'allowed_html' => '<a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>',
+        ],
+      ],
+
+      // Tests that the transformed value for 'allowed_html' is altered when the
+      // destination is filter_html.
+
+      // Test with an empty source array.
+      [
+        [],
+        'filter_html',
+        [],
+      ],
+      // Test with a source string.
+      [
+        'a string',
+        'filter_html',
+        'a string',
+      ],
+      [
+        [
+          'allowed_html' => '<a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>',
+        ],
+        'filter_html',
+        [
+          'allowed_html' => '<a href hreflang> <em> <strong> <cite> <code> <ul type> <ol start type> <li> <dl> <dt> <dd>',
+        ],
+      ],
+    ];
+  }
+
+}