diff -u b/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
--- b/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -849,11 +849,11 @@
  *   The JavaScript/CSS libraries provided by $module. Keyed by internal library
  *   name and passed by reference.
  * @param $extension
- *   The name of the extension that registered the libraries.
+ *   The name of the extension that registered a library.
  */
 function hook_library_info_alter(&$libraries, $extension) {
   // Update Farbtastic to version 2.0.
-  if ($extension == 'core' && isset($libraries['jquery.farbtastic'])) {
+  if ($module == 'core' && isset($libraries['jquery.farbtastic'])) {
     // Verify existing version is older than the one we are updating to.
     if (version_compare($libraries['jquery.farbtastic']['version'], '2.0', '<')) {
       // Update the existing Farbtastic to version 2.0.