diff --git a/library/HTMLPurifier/Config.php b/library/HTMLPurifier/Config.php
index f37cf371..f34a5361 100644
--- a/library/HTMLPurifier/Config.php
+++ b/library/HTMLPurifier/Config.php
@@ -890,7 +890,7 @@ class HTMLPurifier_Config
             // zip(tail(trace), trace) -- but PHP is not Haskell har har
             for ($i = 0, $c = count($trace); $i < $c - 1; $i++) {
                 // XXX this is not correct on some versions of HTML Purifier
-                if ($trace[$i + 1]['class'] === 'HTMLPurifier_Config') {
+                if (isset($trace[$i + 1]['class']) && $trace[$i + 1]['class'] === 'HTMLPurifier_Config') {
                     continue;
                 }
                 $frame = $trace[$i];