diff --git a/CRM/Twingle/Submission.php b/CRM/Twingle/Submission.php
index 469af34..ddd239e 100644
--- a/CRM/Twingle/Submission.php
+++ b/CRM/Twingle/Submission.php
@@ -107,7 +107,10 @@ class CRM_Twingle_Submission {
 
     // Validate custom fields parameter, if given.
     if (!empty($params['custom_fields'])) {
-      if (!is_array($custom_fields = json_decode($params['custom_fields'], TRUE))) {
+      if (is_string($params['custom_fields'])) {
+          $params['custom_fields'] = json_decode($params['custom_fields'], TRUE);
+      }
+      if (!is_array($params['custom_fields'])) {
         throw new CiviCRM_API3_Exception(
           E::ts('Invalid format for custom fields.'),
           'invalid_format'