In a SQL UPDATE statement, how to i "set" a field to empty? (because it was occupied before).

i get a syntax error (missing operator) that points to my sql string

this is the update command i am using but its not working...

Code:
"UPDATE trips set billedamount = '', billstatus = 'unbilled' WHERE tripid = " & DataGridView1.CurrentRow.Cells("tripid").Value.ToString & ""
as you can see, all i want to do is make the billstatus field = "unbilled"
and make the "billedamount" field = nothing(empty/null/whatever)

any help is appreciated