diff --git a/includes/commerce_sos.payment.inc b/includes/commerce_sos.payment.inc
index e56d237..8f8074f 100644
--- a/includes/commerce_sos.payment.inc
+++ b/includes/commerce_sos.payment.inc
@@ -57,7 +57,7 @@ function _commerce_sos_add_payment_fields(array $form, array &$form_state) {
 
         // Establish defaults for the amount if possible.
         if ($balance = commerce_payment_order_balance($form_state['commerce_order'])) {
-          $default_amount = $balance['amount'] > 0 ? $balance['amount'] : '';
+          $default_amount = $balance['amount'] > 0 ? $balance['amount'] : 0;
           $default_currency_code = $balance['currency_code'];
 
           // Convert the default amount to an acceptable textfield value.