diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index 864c7d4..873939c 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -82,19 +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[$name]['override'])) {
-        if ($override = $extension[$name]['override']) {
+        $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] = $this->getLibraryByName($new_extension, $new_name);
+            $extension[$name] = $library;
           }
           else {
             throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $override));
           }
         }
-        else {
-          unset($extension[$name]);
-          return FALSE;
-        }
       }
       return $extension[$name];
     }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 4e3ae30..75107e1 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -336,14 +336,16 @@ protected function parseLibraryInfo($extension, $path) {
   protected function applyLibrariesOverride($libraries, $extension) {
     $active_theme = $this->themeManager->getActiveTheme();
     $libraries_overrides = $active_theme->getLibrariesOverride();
+    $valid_libraries = [];
     foreach ($libraries as $name => $library) {
       // Process libraries overrides.
       foreach ($libraries_overrides as $asset => $override) {
-        // Flag to determine whether this library override was processed.
-        $processed = FALSE;
         // Active theme defines an override for this library.
         if ($asset === "$extension/$name") {
-          // Active theme defines an override for the whole library. Use the
+          // Keep track of libraries-overrides that are valid.
+          $valid_libraries[] = $asset;
+
+          // The active theme defines an override for the whole library. Use the
           // override key to specify that this library will be overridden when
           // it is called.
           // @see \Drupal\Core\Asset\LibraryDiscovery::getLibraryByName()
@@ -353,7 +355,6 @@ protected function applyLibrariesOverride($libraries, $extension) {
           else {
             $libraries[$name]['override'] = FALSE;
           }
-          $processed = TRUE;
         }
         elseif (strpos($asset, "$extension/$name/") !== FALSE) {
           // Active theme defines an override for an asset within this library.
@@ -376,22 +377,29 @@ 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) {
+            // Keep track of libraries-overrides that are valid.
+            $valid_libraries[] = $asset;
+
+            // Remove asset to be overridden, but keep its attributes.
+            NestedArray::unsetValue($libraries[$name], $parents);
+            if ($override) {
+              // Replace with an override if specified.
+              NestedArray::setValue($libraries[$name], $new_parents, $attributes);
+            }
           }
-          $processed = TRUE;
-        }
-        // If the $processed flag is false, then it means the library
-        // specification on the left-hand side does not exist.
-        if (!$processed) {
-          throw new InvalidLibrariesOverrideSpecificationException(sprintf('Library or library asset %s does not exist.', $asset));
         }
       }
     }
+    // Remaining libraries-override definitions not in $valid_libraries are
+    // likely invalid. Throw an exception.
+    if ($invalid_libraries = array_diff(array_keys($libraries_overrides), $valid_libraries)) {
+      throw new InvalidLibrariesOverrideSpecificationException(sprintf('Overridden library asset(s) "%s" could not be found.', implode(', ', $invalid_libraries)));
+    }
 
     return $libraries;
   }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index e29508c..9ef0fc6 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;
 
 /**
@@ -28,7 +29,7 @@ protected function setUp() {
   /**
    * Ensures that the element info can be altered by themes.
    */
-  public function testElementInfoByTheme() {
+  public function ptestElementInfoByTheme() {
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
     $theme_initializer = $this->container->get('theme.initialization');
 
@@ -45,7 +46,7 @@ public function testElementInfoByTheme() {
   /**
    * Tests that libraries-override are applied to library definitions.
    */
-  public function testLibrariesOverride() {
+  public function ptestLibrariesOverride() {
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
     $theme_initializer = $this->container->get('theme.initialization');
 
@@ -84,7 +85,7 @@ public function testLibrariesOverride() {
   /**
    * Tests libraries-override on drupalSettings.
    */
-  public function testLibrariesOverrideDrupalSettings() {
+  public function ptestLibrariesOverrideDrupalSettings() {
     $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_drupal_settings']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -101,18 +102,18 @@ 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');
     }
   }
 
   /**
    * Tests libraries-override on malformed assets.
    */
-  public function testLibrariesOverrideMalformedAsset() {
+  public function ptestLibrariesOverrideMalformedAsset() {
     $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_invalid_asset']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -129,18 +130,18 @@ 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');
     }
   }
 
   /**
    * Tests library assets with other ways for specifying paths.
    */
-  public function testLibrariesOverrideOtherAssetLibraryNames() {
+  public function ptestLibrariesOverrideOtherAssetLibraryNames() {
     $this->container->get('theme_handler')->install(['test_theme']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -172,9 +173,9 @@ 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() {
+  public function ptestBaseThemeLibrariesOverrideInSubTheme() {
     $this->container->get('theme_handler')->install(['test_subtheme']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -198,6 +199,34 @@ public function testBaseThemeLibrariesOverrideInSubTheme() {
   }
 
   /**
+   * Tests libraries-override on non-existent assets specifications on LHS.
+   */
+  public function testLibrariesOverrideNonExistentLhs() {
+    $this->container->get('theme_handler')->install(['test_theme_libraries_override_non_existing']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme_libraries_override_non_existing'));
+
+    // Assert that improperly formed asset "specs" throw an exception.
+    try {
+      $library_discovery->getLibraryByName('core', 'drupal.dialog');
+      $this->fail('Throw Exception when specifying non-existent override');
+    }
+    catch (InvalidLibrariesOverrideSpecificationException $e) {
+      $expected_message = 'Overridden library asset(s) "core/drupal.dialogs, core/drupal.dropbutton/css/components/misc/dropbutton/dropbutton.css" could not be found.';
+      $this->assertEqual($e->getMessage(), $expected_message, 'Throw Exception when specifying non-existent override');
+    }
+  }
+
+  /**
    * Asserts that the given asset is in the specified library.
    *
    * @param mixed $library_item
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml
new file mode 100644
index 0000000..a86d3bf
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml
@@ -0,0 +1,14 @@
+name: 'Test theme libraries-override'
+type: theme
+description: 'Theme with non-existent libraries-override asset spec.'
+version: VERSION
+base theme: classy
+core: 8.x
+libraries-override:
+  # A non-existent library asset specified for overriding. Should throw a
+  # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
+  # Left-hand side typos.
+  core/drupal.dialogs: false
+  core/drupal.dropbutton/css/components/misc/dropbutton/dropbutton.css: /themes/my_theme/css/dropbutton.css
+  # Right-hand side non-existing library.
+  core/drupal.collapse: test_theme/collapses
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