diff --git a/db/routines/vn/procedures/ticket_close.sql b/db/routines/vn/procedures/ticket_close.sql
index f8bbe239b..639b6b505 100644
--- a/db/routines/vn/procedures/ticket_close.sql
+++ b/db/routines/vn/procedures/ticket_close.sql
@@ -85,7 +85,7 @@ BEGIN
 
 		IF(vHasDailyInvoice) AND vHasToInvoice THEN
 			SELECT invoiceSerial(vClientFk, vCompanyFk, 'quick') INTO vSerial;
-			IF NOT vSerial THEN 
+			IF vSerial IS NULL THEN 
 				CALL util.throw('Cannot booking without a serial');
 			END IF;