IsDBNull giver fejl.

Hej,

Jeg bruger nedenstående kode, men det giver en periodisk (ca. hver 2-3 gang) med fejlmeddelelsen:

Cast from type 'DBNull' to type 'String' is not valid.

- hvorfor ????


    Public Function getValue(ByVal strStyle, ByVal strName)
        Dim strContent As String
        Dim objConn As New OleDbConnection(strConn)
        Dim strSQL As String = "SELECT " + strStyle + " FROM tblCssContentFont WHERE strName='" + strName + "';"
        Dim objCmd As New OleDbCommand(strSQL, objConn)
        objConn.Open()
        Dim rdr As String = objCmd.ExecuteScalar()
        If Not IsDBNull(rdr) Then
            Response.Write(rdr)
        End If
        objConn.Close()
    End Function