diff --git a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/NodeReference.php b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/NodeReference.php
index f2f3542..3735a76 100644
--- a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/NodeReference.php
+++ b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/NodeReference.php
@@ -68,9 +68,11 @@ public function transformFieldInstanceSettings(Row $row) {
     $settings['handler'] = 'default:node';
     $settings['handler_settings']['target_bundles'] = [];
 
-    $node_types = array_filter($source_settings['referenceable_types']);
-    if (!empty($node_types)) {
-      $settings['handler_settings']['target_bundles'] = $this->lookupMigrations($this->nodeTypeMigration, $node_types);
+    if (isset($source_settings['referenceable_types'])) {
+      $node_types = array_filter($source_settings['referenceable_types']);
+      if (!empty($node_types)) {
+        $settings['handler_settings']['target_bundles'] = $this->lookupMigrations($this->nodeTypeMigration, $node_types);
+      }
     }
     return $settings;
   }
diff --git a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/TaxonomyTermReference.php b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/TaxonomyTermReference.php
index 7581e2e..ae8abec 100644
--- a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/TaxonomyTermReference.php
+++ b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/TaxonomyTermReference.php
@@ -7,7 +7,7 @@
 /**
  * @MigrateCckField(
  *   id = "taxonomy_term_reference",
- *   core = {6,7},
+ *   core = {7},
  *   type_map = {
  *     "taxonomy_term_reference" = "entity_reference"
  *   }
diff --git a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/UserReference.php b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/UserReference.php
index 25afe01..caf887a 100644
--- a/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/UserReference.php
+++ b/core/lib/Drupal/Core/Field/Plugin/migrate/cckfield/UserReference.php
@@ -71,10 +71,12 @@ public function transformFieldInstanceSettings(Row $row) {
     $settings['handler_settings']['filter']['type'] = '_none';
     $settings['handler_settings']['target_bundles'] = NULL;
 
-    $roles = array_filter($source_settings['referenceable_roles']);
-    if (!empty($roles)) {
-      $settings['handler_settings']['filter']['type'] = 'role';
-      $settings['handler_settings']['filter']['role'] = $this->lookupMigrations($this->userRoleMigration, $roles);
+    if (isset($source_settings['referenceable_roles'])) {
+      $roles = array_filter($source_settings['referenceable_roles']);
+      if (!empty($roles)) {
+        $settings['handler_settings']['filter']['type'] = 'role';
+        $settings['handler_settings']['filter']['role'] = $this->lookupMigrations($this->userRoleMigration, $roles);
+      }
     }
 
     return $settings;
diff --git a/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
index 7e8f23e..d701f3f 100644
--- a/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
+++ b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
@@ -119,8 +119,8 @@ public function testFieldInstanceMigration() {
 
     // Test user reference to entity reference migration.
     $field = FieldConfig::load('node.story.field_user_reference');
-    $this->assertIdentical('User reference', $field->label());
-    $this->assertIdentical('default:user', $field->getSetting('handler'));
+    $this->assertSame('User reference', $field->label());
+    $this->assertSame('default:user', $field->getSetting('handler'));
     $expected = [
       'include_anonymous' => FALSE,
       'filter' => [
diff --git a/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldWidgetSettingsTest.php b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldWidgetSettingsTest.php
index a905d7d..2721406 100644
--- a/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldWidgetSettingsTest.php
+++ b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldWidgetSettingsTest.php
@@ -103,7 +103,7 @@ public function testWidgetSettings() {
     $expected = [];
     $expected['weight'] = 10;
     $expected['type'] = 'datetime_default';
-    $expected['settings'] = array();
+    $expected['settings'] = [];
     $expected['third_party_settings'] = [];
     $this->assertIdentical($expected, $component);
 
@@ -111,7 +111,7 @@ public function testWidgetSettings() {
     $expected = [];
     $expected['weight'] = 11;
     $expected['type'] = 'datetime_default';
-    $expected['settings'] = array();
+    $expected['settings'] = [];
     $expected['third_party_settings'] = [];
     $this->assertIdentical($expected, $component);
 
@@ -119,7 +119,7 @@ public function testWidgetSettings() {
     $expected = [];
     $expected['weight'] = 12;
     $expected['type'] = 'datetime_default';
-    $expected['settings'] = array();
+    $expected['settings'] = [];
     $expected['third_party_settings'] = [];
     $this->assertIdentical($expected, $component);
   }
diff --git a/core/modules/link/src/Plugin/migrate/cckfield/LinkField.php b/core/modules/link/src/Plugin/migrate/cckfield/LinkField.php
index 1d62c7d..a86183c 100644
--- a/core/modules/link/src/Plugin/migrate/cckfield/LinkField.php
+++ b/core/modules/link/src/Plugin/migrate/cckfield/LinkField.php
@@ -51,10 +51,17 @@ public function processCckFieldValues(MigrationInterface $migration, $field_name
    */
   public function transformFieldInstanceSettings(Row $row) {
     $field_settings = $row->getSourceProperty('global_settings');
-    // D6 has optional, required, value and none. D8 only has disabled (0)
-    // optional (1) and required (2).
-    $map = ['disabled' => 0, 'optional' => 1, 'required' => 2];
-    $settings['title'] = $map[$field_settings['title']];
+    if (isset($field_settings['title'])) {
+      // D6 has optional, required, value and none. D8 only has disabled (0)
+      // optional (1) and required (2).
+      $map = ['disabled' => 0, 'optional' => 1, 'required' => 2];
+      $settings['title'] = $map[$field_settings['title']];
+    }
+    else {
+      // In case we are missing title in field settings, use disabled value "0"
+      // as a default value.
+      $settings['title'] = 0;
+    }
     return $settings;
   }