diff --git a/api/v3/TwingleDonation/Submit.php b/api/v3/TwingleDonation/Submit.php
index 4e3e6dd..3701275 100644
--- a/api/v3/TwingleDonation/Submit.php
+++ b/api/v3/TwingleDonation/Submit.php
@@ -304,11 +304,14 @@ 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.
+      // Include user_extrafield and purpose 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'] = $params['user_extrafield'];
       }
+      if(!empty($params['purpose']) && isset($custom_field_mapping['purpose'])) {
+        $params['custom_fields']['purpose'] = $params['purpose'];
+      }
 
       foreach ($params['custom_fields'] as $twingle_field => $value) {
         if (isset($custom_field_mapping[$twingle_field])) {