Fix CRM-12013.

CRM/Export/BAO/Export.php

@@ -996,16 +996,20 @@ INSERT INTO {$componentTable} SELECT distinct gc.contact_id FROM civicrm_group_c
         //build header only once
         $setHeader = FALSE;
 
-        // add payment related information
-        if ($paymentFields) {
-          $paymentData = CRM_Utils_Array::value($row[$paymentTableId], $paymentDetails);
-          if (!is_array($paymentData) || empty($paymentData)) {
-            $paymentData = $nullContributionDetails;
+        // If specific payment fields have been selected for export, payment
+        // data will already be in $row. Otherwise, add payment related 
+        // information, if appropriate.
+        if (!$selectedPaymentFields) {
+          if ($paymentFields) {
+            $paymentData = CRM_Utils_Array::value($row[$paymentTableId], $paymentDetails);
+            if (!is_array($paymentData) || empty($paymentData)) {
+              $paymentData = $nullContributionDetails;
+            }
+            $row = array_merge($row, $paymentData);
           }
-          $row = array_merge($row, $paymentData);
-        }
         elseif (!empty($paymentDetails)) {
-          $row = array_merge($row, $nullContributionDetails);
+            $row = array_merge($row, $nullContributionDetails);
+          }
         }
 
         //remove organization name for individuals if it is set for current employer