hi..
my system should select the maximum QuestionNum from one of the my table in the database. after all, i would like to increment the value QuestionNum to 1...
but i keep on receiving error
"Cast from type 'DBNull' to type 'String' is not valid."

this is my code...

Public Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
If Not IsPostBack Then
Dim MyConnection As SqlConnection
MyConnection = New SqlConnection("server=localhost;database=xxxxxxx;Trusted_Connection=yes")
MyConnection.Open()
Dim sql As String
//Select the max QuestionNum where SurveyID=SurveyID
sql = "SELECT MAX(QuestionNum) FROM Question WHERE SurveyID='" & Session("sSurveyID") & "'"
Dim cmd As New SqlCommand(sql, MyConnection)
Dim QuestionNum As Integer
Dim id As String
id = cmd.ExecuteScalar
QuestionNum = id
MyConnection.Close()
End If
End Sub


p/s: can anyone explain what is wrong here?