diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 2af07a2..367dfde 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -381,8 +381,7 @@ function _theme($hook, $variables = array()) {
       $output = SafeMarkup::set($info['function']($variables));
     }
     else {
-      // The function is registered but does not exist (yet).
-      trigger_error(sprintf("Missing theme function '%s' for theme hook '%s'. Either rebuild the theme registry and/or correct the definition.", $info['function'], $hook), E_USER_WARNING);
+      trigger_error(sprintf("Missing theme function '%s' for theme hook '%s'.", $info['function'], $hook), E_USER_ERROR);
     }
   }
   else {