diff --git a/support/doctype/support_ticket/support_ticket.py b/support/doctype/support_ticket/support_ticket.py
index e7f3077a560..fbab2b71d53 100644
--- a/support/doctype/support_ticket/support_ticket.py
+++ b/support/doctype/support_ticket/support_ticket.py
@@ -137,5 +137,5 @@ class DocType(TransactionBase):
 		update_feed(self.doc)
 		
 	def on_trash(self):
-		webnotes.conn.sql("""update `tabCommunication set support_ticket="" 
-			where support_ticket=%s`""", self.doc.name)
+		webnotes.conn.sql("""update `tabCommunication` set support_ticket=NULL 
+			where support_ticket=%s""", (self.doc.name,))