diff --git a/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesOverrideSpecificationException.php b/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesOverrideSpecificationException.php
new file mode 100644
index 0000000..89667aa
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesOverrideSpecificationException.php
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
+ */
+
+namespace Drupal\Core\Asset\Exception;
+
+/**
+ * Defines a custom exception if a definition refers to a non-existent library.
+ */
+class InvalidLibrariesOverrideSpecificationException extends \RuntimeException {
+
+}
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index bcea7f4..873939c 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Asset;
 
+use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Cache\CacheCollectorInterface;
 use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
 
@@ -81,14 +82,23 @@ public function getLibraryByName($extension, $name) {
       // Handle libraries that are marked for override or removal.
       // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
       if (isset($extension[$name]['override'])) {
-        if ($extension[$name]['override']) {
-          list($new_extension, $new_name) = explode('/', $extension[$name]['override']);
-          $extension[$name] = $this->getLibraryByName($new_extension, $new_name);
-        }
-        else {
+        $override = $extension[$name]['override'];
+        if ($override === FALSE) {
+          // Remove the library definition if false is given.
           unset($extension[$name]);
           return FALSE;
         }
+        else {
+          // Otherwise replace with existing library definition if it exists.
+          // Throw an exception if it doesn't.
+          list($new_extension, $new_name) = explode('/', $override);
+          if ($library = $this->getLibraryByName($new_extension, $new_name)) {
+            $extension[$name] = $library;
+          }
+          else {
+            throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $override));
+          }
+        }
       }
       return $extension[$name];
     }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index b861ec6..f6de366 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Asset;
 
 use Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException;
+use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Asset\Exception\InvalidLibraryFileException;
 use Drupal\Core\Asset\Exception\LibraryDefinitionMissingLicenseException;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -335,13 +336,12 @@ protected function parseLibraryInfo($extension, $path) {
   protected function applyLibrariesOverride($libraries, $extension) {
     $active_theme = $this->themeManager->getActiveTheme();
     $libraries_overrides = $active_theme->getLibrariesOverride();
-    $theme_path = $active_theme->getPath();
     foreach ($libraries as $name => $library) {
       // Process libraries overrides.
       foreach ($libraries_overrides as $asset => $override) {
         // Active theme defines an override for this library.
         if ($asset === "$extension/$name") {
-          // Active theme defines an override for the whole library. Use the
+          // The active theme defines an override for the whole library. Use the
           // override key to specify that this library will be overridden when
           // it is called.
           // @see \Drupal\Core\Asset\LibraryDiscovery::getLibraryByName()
@@ -356,12 +356,12 @@ protected function applyLibrariesOverride($libraries, $extension) {
           // Active theme defines an override for an asset within this library.
           // Throw an exception if the asset is not properly specified.
           if (substr_count($asset, '/') < 3) {
-            throw new \LogicException(sprintf('Library asset %s is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', $asset));
+            throw new InvalidLibrariesOverrideSpecificationException(sprintf('Library asset %s is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', $asset));
           }
           list(, , $sub_key, $value) = explode('/', $asset, 4);
           if ($sub_key === 'drupalSettings') {
             // drupalSettings may not be overridden.
-            throw new \LogicException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', $asset));
+            throw new InvalidLibrariesOverrideSpecificationException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', $asset));
           }
           elseif ($sub_key === 'css') {
             // SMACSS category should be incorporated into the asset name.
@@ -373,12 +373,17 @@ protected function applyLibrariesOverride($libraries, $extension) {
             $parents = [$sub_key, $value];
             $new_parents = [$sub_key, $override];
           }
-          // Remove asset to be overridden, but keep its attributes.
-          $attributes = NestedArray::getValue($libraries[$name], $parents);
-          NestedArray::unsetValue($libraries[$name], $parents);
-          if ($override) {
-            // Replace with an override if specified.
-            NestedArray::setValue($libraries[$name], $new_parents, $attributes);
+          // Get the attributes of the asset to be overridden. If the key does
+          // not exist, then throw an exception.
+          $key_exists = NULL;
+          $attributes = NestedArray::getValue($libraries[$name], $parents, $key_exists);
+          if ($key_exists) {
+            // Remove asset to be overridden, but keep its attributes.
+            NestedArray::unsetValue($libraries[$name], $parents);
+            if ($override) {
+              // Replace with an override if specified.
+              NestedArray::setValue($libraries[$name], $new_parents, $attributes);
+            }
           }
         }
       }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index e29508c..7d7c7b9 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\Asset;
 
+use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\simpletest\KernelTestBase;
 
 /**
@@ -101,11 +102,11 @@ public function testLibrariesOverrideDrupalSettings() {
     // Assert that drupalSettings cannot be overridden and throws an exception.
     try {
       $library_discovery->getLibraryByName('core', 'drupal.ajax');
-      $this->fail('Throw LogicException when trying to override drupalSettings');
+      $this->fail('Throw Exception when trying to override drupalSettings');
     }
-    catch (\LogicException $e) {
+    catch (InvalidLibrariesOverrideSpecificationException $e) {
       $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override core/drupal.ajax/drupalSettings/ajaxPageState. Use hook_library_info_alter() instead.';
-      $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when trying to override drupalSettings');
+      $this->assertEqual($e->getMessage(), $expected_message, 'Throw Exception when trying to override drupalSettings');
     }
   }
 
@@ -129,11 +130,11 @@ public function testLibrariesOverrideMalformedAsset() {
     // Assert that improperly formed asset "specs" throw an exception.
     try {
       $library_discovery->getLibraryByName('core', 'drupal.dialog');
-      $this->fail('Throw LogicException when specifying invalid override');
+      $this->fail('Throw Exception when specifying invalid override');
     }
-    catch (\LogicException $e) {
+    catch (InvalidLibrariesOverrideSpecificationException $e) {
       $expected_message = 'Library asset core/drupal.dialog/css is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".';
-      $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when specifying invalid override');
+      $this->assertEqual($e->getMessage(), $expected_message, 'Throw Exception when specifying invalid override');
     }
   }
 
@@ -172,7 +173,7 @@ public function testLibrariesOverrideOtherAssetLibraryNames() {
   }
 
   /**
-   * Tests that base theme libraries-remove still apply in sub themes.
+   * Tests that base theme libraries-override still apply in sub themes.
    */
   public function testBaseThemeLibrariesOverrideInSubTheme() {
     $this->container->get('theme_handler')->install(['test_subtheme']);
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
index 0939c27..e6e6f1f 100644
--- a/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
+++ b/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
@@ -5,5 +5,6 @@ version: VERSION
 base theme: classy
 core: 8.x
 libraries-override:
-  # drupalSettings libraries override. Should throw a \LogicException.
+  # drupalSettings libraries override. Should throw a
+  # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
   core/drupal.ajax/drupalSettings/ajaxPageState: { }
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
index ecba74e..befe8d8 100644
--- a/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
+++ b/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
@@ -5,5 +5,6 @@ version: VERSION
 base theme: classy
 core: 8.x
 libraries-override:
-  # A malformed library asset name. Should throw a \LogicException.
+  # A malformed library asset name. Should throw a
+  # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
   core/drupal.dialog/css: false