diff --git a/Civi/DataProcessor/DataFlow/MultipleDataFlows/SimpleJoin.php b/Civi/DataProcessor/DataFlow/MultipleDataFlows/SimpleJoin.php
index 1064f61e0825760ebbcef356ad28176151ffac6e..2a24779302d485e1a9b1482d068e693e738fa674 100644
--- a/Civi/DataProcessor/DataFlow/MultipleDataFlows/SimpleJoin.php
+++ b/Civi/DataProcessor/DataFlow/MultipleDataFlows/SimpleJoin.php
@@ -159,7 +159,7 @@ class SimpleJoin implements JoinInterface, SqlJoinInterface {
     }
 
     $defaults = array();
-    if (isset($joinConfiguration['left_prefix'])) {
+    if (isset($joinConfiguration['left_field'])) {
       $defaults['left_field'] = $joinConfiguration['left_field'];
     }
     if (isset($joinConfiguration['right_prefix'])) {