If i want to save my values back into the database i get the following error:invalid use of null if one of my controls are empty.

I've tried everything?
What must i do?

I am using the following querry:

sql = "update Main set " & _
"Job_No = '" & txtJob.Text & "', " & _
"Cust = '" & txtCust.Text & "', " & _
"Site = '" & txtSite.Text & "', " & _
"Order_No = '" & txtOrder.Text & "', " & _
"Model = '" & txtModel.Text & "', " & _
"Prefix = '" & txtPrefix.Text & "', " & _
"Serial = '" & txtSerial.Text & "', " & _
"SMR = '" & txtSMR.Text & "', " & _
"DateOpen = '" & txtDateOpen.Text & "', " & _
"DateClosed = '" & txtDateClosed.Text & "', " & _
"DateStart & = '" & txtDateStart.Text & "', " & _
"DateEnd = '" & txtDateEnd.Text & "' , " & _
"Status = '" & chkStatus.Value & "' , " & _
"Remarks = '" & txtRemarks.Text & "'where idJob = " & _
intCurrJob

'execute the query
dbProgram.Execute sql

End If