Help on DBNULL values

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VB

SQL

Hello,please i am working on a project.In one of the web page there is a textbox to select the total amount between two given periods which the user must give.The code works fine if the given dates are present in the database.But if those date given are not present it throws an exception saying "Object cannot be cast from DBNull to other types". Don't know what to do to make it work.i am still a baby in vb and sql.Any help is highly appreciated.Below is my code

Protected Sub SearchButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles SearchButton.Click



        If TxtEndDate.Text = String.Empty Then
        ElseIf TxtStartDate.Text = String.Empty Then
            Exit Sub
        Else

            Try


                Dim connectionstring As String = ConfigurationManager.ConnectionStrings("LordsBannerMotorsConnectionString1").ConnectionString
                Dim insertsql As String = "SELECT SUM(Amount) As [Total Amount] FROM MoneyPaidToBank WHERE " &
                                         " Bank_Location=@Location AND Date BETWEEN @Start AND @End"
                Using connection As New SqlConnection(connectionstring)
                    connection.Open()

                    Dim mycommand As New SqlCommand(insertsql, connection)
                    mycommand.Parameters.Add("@Location", SqlDbType.VarChar, 50)
                    mycommand.Parameters("@Location").Value = TotalMoneyDropDownList1.SelectedValue
                    mycommand.Parameters.Add("@Start", SqlDbType.SmallDateTime)
                    mycommand.Parameters("@Start").Value = CType(TxtStartDate.Text, Date)
                    mycommand.Parameters.Add("@End", SqlDbType.SmallDateTime)
                    mycommand.Parameters("@End").Value = CType(TxtEndDate.Text, Date)
                    Dim str As String = ""
                    Dim reader As SqlDataReader = mycommand.ExecuteReader
                    If reader.HasRows Then
                        While reader.Read
                            str &= Convert.ToDouble(reader.Item("Total Amount"))

                        End While
                    Else
                        SearchLabel.Text = "Record not found"
                    End If

                    SearchLabel.Text = "The total Amount is " & str

                    connection.Close()
                End Using
            Catch ex As Exception
                SearchLabel.Text = ex.Message

            End Try
        End If