diff -ur original/shipping/uc_flatrate/uc_flatrate.admin.inc patched/shipping/uc_flatrate/uc_flatrate.admin.inc
--- original/shipping/uc_flatrate/uc_flatrate.admin.inc	2011-03-11 18:13:32.000000000 +0200
+++ patched/shipping/uc_flatrate/uc_flatrate.admin.inc	2011-04-07 19:41:45.000000000 +0300
@@ -164,7 +164,9 @@
     variable_set('uc_quote_method_weight', $weight);
 
     // Ensure Rules picks up the new condition.
-    entity_flush_caches();
+    if (module_exists('rules')) {
+      entity_flush_caches();
+    }
 
     drupal_set_message(t("Created and enabled new flat rate shipping method."));
   }