diff --git a/htdocs/projet/card.php b/htdocs/projet/card.php
index 68710622ed1..5745f8590ea 100644
--- a/htdocs/projet/card.php
+++ b/htdocs/projet/card.php
@@ -371,8 +371,10 @@ if (empty($reshook)) {
 					$idoppstatuswon = (int) dol_getIdFromCode($db, 'WON', 'c_lead_status', 'code', 'rowid');
 
 					if ($object->opp_status == $idoppstatuswon) {
-						// Switch the thirdparty into a customer
-						$object->thirdparty->setAsCustomer();
+						// Switch the thirdparty into a customer (only if thirdparty exists)
+						if (!empty($object->thirdparty) && !empty($object->thirdparty->id)) {
+							$object->thirdparty->setAsCustomer();
+						}
 					}
 				}
 			}
@@ -468,8 +470,10 @@ if (empty($reshook)) {
 					$idoppstatuswon = (int) dol_getIdFromCode($db, 'WON', 'c_lead_status', 'code', 'rowid');
 
 					if (!$error && $object->opp_status == $idoppstatuswon) {
-						// Switch the thirdparty into a customer
-						$object->thirdparty->setAsCustomer();
+						// Switch the thirdparty into a customer (only if thirdparty exists)
+						if (!empty($object->thirdparty) && !empty($object->thirdparty->id)) {
+							$object->thirdparty->setAsCustomer();
+						}
 					}
 				}
 			}