diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 1966f18..cda6e6b 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -705,20 +705,21 @@ public function destruct() {
    */
   public function getPrefixGroupedUserFunctions() {
     $functions = get_defined_functions();
-    $seen = array_flip($this->seenFunctions);
 
     // Splitting user defined functions into groups by the first prefix.
     foreach ($functions['user'] as $function) {
-      if (isset($seen[$function])) {
+      if (isset($this->seenFunctions[$function])) {
         continue;
       }
       list($first_prefix,) = explode('_', $function, 2);
       $this->groupedFunctions[$first_prefix][] = $function;
     }
 
-    // If there were any unseen functions, update the list of seen ones.
+    // The theme registry may load new code. On encountering newly defined
+    // functions, we save the list of defined functions again. This works
+    // because functions cannot disappear between calls.
     if (isset($first_prefix)) {
-      $this->seenFunctions = $functions['user'];
+      $this->seenFunctions = array_fill_keys($functions['user'], TRUE);
     }
 
     return $this->groupedFunctions;