diff --git a/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesExtendSpecificationException.php b/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesExtendSpecificationException.php
new file mode 100644
index 0000000..f1b9df0
--- /dev/null
+++ b/core/lib/Drupal/Core/Asset/Exception/InvalidLibrariesExtendSpecificationException.php
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Asset\Exception\InvalidLibrariesExtendSpecificationException.
+ */
+
+namespace Drupal\Core\Asset\Exception;
+
+/**
+ * Defines a custom exception for an invalid libraries-extend specification.
+ */
+class InvalidLibrariesExtendSpecificationException extends \RuntimeException {
+
+}
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
index 8b35226..f7dc640 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Asset;
 
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Asset\Exception\InvalidLibrariesExtendSpecificationException;
 use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Cache\CacheCollector;
 use Drupal\Core\Cache\CacheBackendInterface;
@@ -133,17 +134,39 @@ protected function getLibraryDefinitions($extension) {
     return $libraries;
   }
 
+  /**
+   * Applies the libraries-extend specified by the active theme.
+   *
+   * This extends the library definitions with the those specified by the
+   * libraries-extend specifications for the active theme.
+   *
+   * @param string $extension
+   *   The name of the extension for which library definitions will be extended.
+   * @param string $library_name
+   *   The name of the library whose definitions is to be extended.
+   * @param $library_definition
+   *   The library definition to be extended.
+   *
+   * @return array
+   *   The library definition extended as specified by libraries-extend.
+   *
+   * @throws \Drupal\Core\Asset\Exception\InvalidLibrariesExtendSpecificationException
+   */
   protected function applyLibrariesExtend($extension, $library_name, $library_definition) {
-    if (!empty($this->themeManager->getActiveTheme()->getLibrariesExtend()["$extension/$library_name"])) {
-      $libraries_extend = $this->themeManager->getActiveTheme()->getLibrariesExtend()["$extension/$library_name"];
-      foreach ($libraries_extend as $library_extend_name) {
-        list($new_extension, $new_library_name) = explode('/', $library_extend_name);
+    $active_theme = $this->themeManager->getActiveTheme()->getLibrariesExtend();
+    if (!empty($active_theme["$extension/$library_name"])) {
+      foreach ($active_theme["$extension/$library_name"] as $library_extend_name) {
+        if (!is_string($library_extend_name)) {
+          // Only string library names are allowed.
+          throw new InvalidLibrariesExtendSpecificationException('The libraries-extend specification must be a string.');
+        }
+        list($new_extension, $new_library_name) = explode('/', $library_extend_name, 2);
         $new_libraries = $this->get($new_extension);
         if (isset($new_libraries[$new_library_name])) {
           $library_definition = NestedArray::mergeDeep($library_definition, $new_libraries[$new_library_name]);
         }
         else {
-          throw new InvalidLibrariesOverrideSpecificationException(sprintf('The specified library %s does not exist.', $library_extend_name));
+          throw new InvalidLibrariesExtendSpecificationException(sprintf('The specified library %s does not exist.', $library_extend_name));
         }
       }
     }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryCollectorTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryCollectorTest.php
index 1c3733d..6199438 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryCollectorTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryCollectorTest.php
@@ -94,7 +94,7 @@ public function testResolveCacheMiss() {
     $this->activeTheme = $this->getMockBuilder('Drupal\Core\Theme\ActiveTheme')
       ->disableOriginalConstructor()
       ->getMock();
-    $this->themeManager->expects($this->once())
+    $this->themeManager->expects($this->exactly(3))
       ->method('getActiveTheme')
       ->willReturn($this->activeTheme);
     $this->activeTheme->expects($this->once())
@@ -120,7 +120,7 @@ public function testDestruct() {
     $this->activeTheme = $this->getMockBuilder('Drupal\Core\Theme\ActiveTheme')
       ->disableOriginalConstructor()
       ->getMock();
-    $this->themeManager->expects($this->once())
+    $this->themeManager->expects($this->exactly(3))
       ->method('getActiveTheme')
       ->willReturn($this->activeTheme);
     $this->activeTheme->expects($this->once())