diff --git a/paymentreference/paymentreference.module b/paymentreference/paymentreference.module
index d2108f4..fdfe569 100644
--- a/paymentreference/paymentreference.module
+++ b/paymentreference/paymentreference.module
@@ -526,6 +526,12 @@ function paymentreference_field_access($op, $field, $entity_type, $entity, $acco
  */
 function paymentreference_form_process_paymentreference(array $element, array &$form_state, array $form) {
   $pid = $element['#default_value'];
+
+  // Protect against a bogus string value: https://drupal.org/node/1994392
+  if (!is_numeric($pid)) {
+    $pid = 0;
+  }
+
   if (!$pid) {
     $pid = (int) call_user_func_array($element['#payment_load_callback'], $element['#payment_load_arguments']);
   }