diff --git a/core/lib/Drupal/Core/Config/TypedConfigManager.php b/core/lib/Drupal/Core/Config/TypedConfigManager.php
index 6775aab..e55865e 100644
--- a/core/lib/Drupal/Core/Config/TypedConfigManager.php
+++ b/core/lib/Drupal/Core/Config/TypedConfigManager.php
@@ -135,9 +135,15 @@ public function getDefinition($base_plugin_id, $exception_on_invalid = TRUE) {
     $definition = $definitions[$type];
     // Check whether this type is an extension of another one and compile it.
     if (isset($definition['type'])) {
+      // Replace the extended type's wildcards with the values of the original
+      // type's wildcards.
       $extended_type = preg_replace_callback('/\*/', function() use ($wildcard_values) {
         static $wildcard_index = 0;
-        return $wildcard_values[$wildcard_index++];
+
+        $wildcard_value = isset($wildcard_values[$wildcard_index]) ? $wildcard_values[$wildcard_index] : '*';
+        $wildcard_index++;
+
+        return $wildcard_value;
       }, $definition['type']);
       $merge = $this->getDefinition($extended_type, $exception_on_invalid);
       // Preserve integer keys on merge, so sequence item types can override
diff --git a/core/tests/Drupal/Tests/Core/Config/TypedConfigManagerTest.php b/core/tests/Drupal/Tests/Core/Config/TypedConfigManagerTest.php
new file mode 100644
index 0000000..dc1dafe
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Config/TypedConfigManagerTest.php
@@ -0,0 +1,156 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Config\TypedConfigManagerTest.
+ */
+
+namespace Drupal\Tests\Core\Config;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Config\TypedConfigManager;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\Tests\UnitTestCase;
+
+
+/**
+ * @coversDefaultClass \Drupal\Core\Config\TypedConfigManager
+ *
+ * @group Config
+ */
+class TypedConfigManagerTest extends UnitTestCase {
+
+  /**
+   * The cache backend.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $cacheBackend;
+
+  /**
+   * The configuration storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $configStorage;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $moduleHandler;
+
+  /**
+   * The configuration schema storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $schemaStorage;
+
+  /**
+   * The subject under test.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManager
+   */
+  protected $sut;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->cacheBackend = $this->getMock(CacheBackendInterface::class);
+    $this->configStorage = $this->getMock(StorageInterface::class);
+    $this->moduleHandler = $this->getMock(ModuleHandlerInterface::class);
+    $this->schemaStorage = $this->getMock(StorageInterface::class);
+
+    $this->sut = new TypedConfigManager($this->configStorage, $this->schemaStorage, $this->cacheBackend, $this->moduleHandler);
+  }
+
+  /**
+   * @covers ::getDefinition
+   * @covers ::getFallbackName
+   *
+   * @dataProvider providerGetDefinition
+   *
+   * @param mixed[] $expected_definition
+   *   The expected schema definition.
+   * @param string $schema_id
+   *   The ID of the expected schema.
+   * @param array[] $schema_definitions
+   *   All available schema definitions.
+   */
+  public function testGetDefinition(array $expected_definition, $schema_id, array $schema_definitions) {
+    $this->schemaStorage->expects($this->atLeastOnce())
+      ->method('listAll')
+      ->willReturn([]);
+    $this->schemaStorage->expects($this->atLeastOnce())
+      ->method('readMultiple')
+      ->willReturn([$schema_definitions]);
+
+    $this->assertSame($expected_definition, $this->sut->getDefinition($schema_id));
+  }
+
+  /**
+   * Provides data to self::testGetDefinition().
+   */
+  public function providerGetDefinition() {
+    $undefined_schema_definition = [
+      'definition_class' => DataDefinition::class,
+      'check' => 'Undefined',
+    ];
+    $simple_schema_definition = [
+      'definition_class' => DataDefinition::class,
+      'check' => 'Simple',
+    ];
+    $fallback_schema_definition = [
+      'definition_class' => DataDefinition::class,
+      'check' => 'Fallback',
+    ];
+    $fallback_resolved_schema_definition = [
+      'definition_class' => DataDefinition::class,
+      'check' => 'FallbackResolved',
+    ];
+    $fallback_alias_schema_definition = [
+      'type' => 'fallback.*',
+      // We do not need a check item, because this is an alias.
+    ];
+    $schema_definitions = [
+      'simple' => $simple_schema_definition,
+      'undefined' => $undefined_schema_definition,
+      'fallback.*' => $fallback_schema_definition,
+      'fallback.resolved' => $fallback_resolved_schema_definition,
+      'fallback_alias.*' => $fallback_alias_schema_definition,
+    ];
+
+    return [
+      // A non-existent schema ID.
+      [$undefined_schema_definition + [
+          'type' => 'undefined',
+        ], 'non_existent', $schema_definitions],
+      // A schema with an ID without wildcards.
+      [$simple_schema_definition + [
+          'type' => 'simple',
+        ], 'simple', $schema_definitions],
+      // A non-existent schema ID for which a direct fallback exists.
+      [$fallback_schema_definition + [
+          'type' => 'fallback.*',
+        ], 'fallback.non_existent', $schema_definitions],
+      // A schema ID for which a direct, unused fallback exists.
+      [$fallback_resolved_schema_definition + [
+          'type' => 'fallback.resolved',
+        ], 'fallback.resolved', $schema_definitions],
+      // An aliased schema ID.
+      [$fallback_schema_definition + [
+          'type' => 'fallback_alias.*',
+        ], 'fallback_alias.*', $schema_definitions],
+      // An aliased schema ID with a resolved wildcard.
+      [$fallback_resolved_schema_definition + [
+          'type' => 'fallback_alias.*',
+        ], 'fallback_alias.resolved', $schema_definitions],
+    ];
+  }
+
+}