Index: includes/handlers/pay.inc
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/pay/includes/handlers/pay.inc,v
retrieving revision 1.25
diff -u -r1.25 pay.inc
--- includes/handlers/pay.inc	28 Sep 2010 05:46:22 -0000	1.25
+++ includes/handlers/pay.inc	29 Sep 2010 04:48:33 -0000
@@ -218,7 +218,7 @@
       'pay-'. $handler,
       'pay-'. $form_type .'-'. $form_state['pay_form_type'],
     );
-    $form['#attributes']['class'] .= join(' ', $class);
+    $form['#attributes']['class'] = empty($form['#attributes']['class']) ? join(' ', $class) : $form['#attributes']['class'] . ' ' . join(' ', $class);
 
     $form['#pay'][] = $values;