diff --git a/CRM/Report/Form/Activity.php b/CRM/Report/Form/Activity.php
index a2c95fe8a59f..cc4752b39b90 100644
--- a/CRM/Report/Form/Activity.php
+++ b/CRM/Report/Form/Activity.php
@@ -697,7 +697,9 @@ public function add2group($groupID) {
 GROUP BY civicrm_activity_id $having {$this->_orderBy}";
     $select = 'AS addtogroup_contact_id';
     $query = str_ireplace('AS civicrm_contact_contact_target_id', $select, $query);
+    CRM_Core_DAO::disableFullGroupByMode();
     $dao = $this->executeReportQuery($query);
+    CRM_Core_DAO::reenableFullGroupByMode();
 
     $contactIDs = array();
     // Add resulting contacts to group