Conversion error DBNull cannot be converted to long

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VB

ASP.NET4

Hello all, thanks for reading my post. I have the following code and every so often I get a conversion error: DBNull cannot be converted to long. I am using the following code:

Protected Sub Calendar332_SelectionChanged(sender As Object, e As EventArgs)
        'declare SQL statement that will query the database
        Dim sqlstring As String = "SELECT firstname, lastname, startdate, starttime, endtime, COALESCE(hours,0) AS hours FROM hoursworked WHERE startdate >= '" & Calendar332.SelectedDate & "' AND startdate <= '" & Calendar333.SelectedDate & "' ORDER BY startdate"
        

        Dim strSQLconnection As String = "Data Source=SQL Server;SERVER=123456; Initial Catalog=ABC; UID=abc123;PWD=123;"

        Dim sqlConnection As New Data.SqlClient.SqlConnection(strSQLconnection)
        Dim sqlCommand As New Data.SqlClient.SqlCommand(sqlstring, sqlConnection)

        sqlConnection.Open()
        Dim reader As Data.SqlClient.SqlDataReader = sqlCommand.ExecuteReader()

        GridView7.Visible = True
        GridView7.DataSource = reader
        GridView7.DataBind()
        reader.Close()
        Dim totalhours
        Dim sqlstring1 As String = "SELECT firstname, lastname, SUM(hours) AS totalhours FROM hoursworked WHERE startdate >= '" & Calendar332.SelectedDate & "' AND startdate <= '" & Calendar333.SelectedDate & "'  group by firstname, lastname"
        Dim sqlConnection1 As New Data.SqlClient.SqlConnection(strSQLconnection)
        Dim sqlCommand1 As New Data.SqlClient.SqlCommand(sqlstring1, sqlConnection1)
        '  Dim sqlCommand1 As New Data.SqlClient.SqlCommand(sqlstring1, sqlConnection)
        sqlConnection1.Open()
        Dim reader1 As Data.SqlClient.SqlDataReader = sqlCommand1.ExecuteReader()

        If totalhours Is DBNull.Value Then
            totalhours = 0
        End If


        GridView8.Visible = True
        GridView8.DataSource = reader1
        GridView8.DataBind()
        reader1.Close()
    End Sub

In the aspx part I have:

<ItemTemplate>
                        <%# Format((CLng(DataBinder.Eval(Container.DataItem, "totalhours")) - (CLng(DataBinder.Eval(Container.DataItem, "totalhours")) Mod 60)) / 60, "0")
                        %>
                        :
                        <%# Format(CLng(DataBinder.Eval(Container.DataItem, "totalhours")) Mod 60, "00")%>
                        </ItemTemplate>

After I used COALESCE in the first select statement the error went away. But, when I act as an end user and start clicking on stuff randomly the error returned. I was hoping my if statement would do the trick, but no such luck. Any ideas or suggestions would be greatly appreciated.

Thank you very much,
D-Bar