diff --git a/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryDefinitionException.php b/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryDefinitionException.php
new file mode 100644
index 0000000..9f89e21
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryDefinitionException.php
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\Exception\InvalidLibraryDefinitionException.
+ */
+
+namespace Drupal\Core\Asset\Exception;
+
+/**
+ * Defines a custom exception if a definition refers to a non-existent library.
+ */
+class InvalidLibraryDefinitionException extends \RuntimeException {
+
+}
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index bcea7f4..55fe5fd 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\InvalidLibraryDefinitionException;
 use Drupal\Core\Cache\CacheCollectorInterface;
 use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
 
@@ -81,9 +82,14 @@ 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, $new_name);
+        if ($override = $extension[$name]['override']) {
+          list($new_extension, $new_name) = explode('/', $override);
+          if ($library = $this->getLibraryByName($new_extension, $new_name)) {
+            $extension[$name] = $this->getLibraryByName($new_extension, $new_name);
+          }
+          else {
+            throw new InvalidLibraryDefinitionException(sprintf('The specified library %s does not exist.', $override));
+          }
         }
         else {
           unset($extension[$name]);
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index b861ec6..1670f16 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\InvalidLibraryDefinitionException;
 use Drupal\Core\Asset\Exception\InvalidLibraryFileException;
 use Drupal\Core\Asset\Exception\LibraryDefinitionMissingLicenseException;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -335,10 +336,11 @@ 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) {
+        // 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
@@ -351,6 +353,7 @@ 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.
@@ -380,6 +383,12 @@ protected function applyLibrariesOverride($libraries, $extension) {
             // Replace with an override if specified.
             NestedArray::setValue($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 InvalidLibraryDefinitionException(sprintf('Library or library asset %s does not exist.', $asset));
         }
       }
     }