diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 7d52c59..8520eda 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -247,8 +247,10 @@ protected function resolveStyleSheetPlaceholders($css_file) {
 
     $token = substr($token_candidate, 1);
 
+    // Prime extensions.
+    $this->getExtensions();
     if (isset($this->extensions[$token])) {
-      return str_replace($token_candidate, $this->getExtensions()[$token]->getPath(), $css_file);
+      return str_replace($token_candidate, $this->extensions[$token]->getPath(), $css_file);
     }
   }
 }