Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.756.2.83
diff -u -r1.756.2.83 common.inc
--- includes/common.inc	11 May 2010 09:43:18 -0000	1.756.2.83
+++ includes/common.inc	13 May 2010 08:13:05 -0000
@@ -3437,7 +3437,8 @@
     }
 
     // Build arrays for the fields, placeholders, and values in our query.
-    if (isset($object->$field)) {
+    // For updates, don't include the primary keys in the fields.
+    if (isset($object->$field) && !in_array($field, $update)) {
       $fields[] = $field;
       $placeholders[] = db_type_placeholder($info['type']);