fixes #624

lib/Doctrine/Connection.php

@@ -544,7 +544,7 @@ abstract class Doctrine_Connection extends Doctrine_Configurable implements Coun
         foreach ($fields as $fieldName => $value) {
             if ($value instanceof Doctrine_Expression) {
                 $set[] = $table->getColumnName($fieldName) . ' = ' . $value->getSql();
-                unset($fields[$name]);
+                unset($fields[$fieldName]);
             } else {
                 $set[] = $table->getColumnName($fieldName) . ' = ?';
             }