diff --git a/api/v3/TwingleDonation/Submit.php b/api/v3/TwingleDonation/Submit.php
index 22d2355..4e3e6dd 100644
--- a/api/v3/TwingleDonation/Submit.php
+++ b/api/v3/TwingleDonation/Submit.php
@@ -304,6 +304,12 @@ function civicrm_api3_twingle_donation_Submit($params) {
     if (!empty($params['custom_fields'])) {
       $custom_field_mapping = $profile->getCustomFieldMapping();
 
+      // Include user_extrafield in custom_field_mapping if it is referenced there.
+      // See issue #50.
+      if(!empty($params['user_extrafield']) && isset($custom_field_mapping['user_extrafield'])) {
+        $params['custom_fields']['user_extrafield'] = $params['user_extrafield'];
+      }
+
       foreach ($params['custom_fields'] as $twingle_field => $value) {
         if (isset($custom_field_mapping[$twingle_field])) {
           // Get custom field definition to store values by entity the field