--- imagecache.module.orig	2008-11-27 14:57:23.000000000 +0100
+++ imagecache.module	2008-11-27 14:58:35.000000000 +0100
@@ -823,8 +823,9 @@
     return $presets;
   }
 
-  // Grab from cache or build the array.
-  if ($cache = cache_get('imagecache:presets', 'cache')) {
+  // Grab from cache or build the array. To ensure that the Drupal 5 upgrade
+  // path works, we also check whether the presets list is an array.
+  if ($cache = cache_get('imagecache:presets', 'cache') && is_array($cache->data)) {
     $presets = $cache->data;
   }
   else {