Invalid use of null error

Hi guys

I am getting an error message at the following line of code. Any help would be much appreciated.

strsql = "select sum(cases) as [Total Letters] from tblmain where tdate >= #" & Format(DateSerial(ComboBox3, ComboBox2, ComboBox1), "mm/dd/yyyy") & "# and tdate <= #" & Format(DateSerial(ComboBox6, ComboBox5, ComboBox4), "mm/dd/yyyy") & "# AND type='GH'"
rs.Open strsql, cn

  Label5.Caption = rs.Fields(0).Value 
rs.Close

Thanks
Aman